This is a big one, ya'll. We put a lot of thought into this story so there is a lot of background information and a moderately thorough timeline for the story itself. Of course there are a few loose ends for you to work out for yourself if you do decide to do something with the idea, but that's half the fun of writing fanfiction if you ask me! If you have any questions or just wanna talk an idea over, I always answer PMs :)
This story starts about two years after the Battle of Hogwarts with Harry sitting alone in a Ministry courtroom, waiting for the last day of Fenrir Greyback's trial to start. He reflects on all the things that have changed: Ron and Hermione dating, Hermione is back at Hogwarts and about to graduate, Ron in Auror training, Ginny and he are broken up, which he's still not really okay with. He's an Auror, but spends most of his time with the trials these days, though Greyback's is the last main one he'll be involved in. (Important to note: he has all the Hallows on him in this scene, in a little Undetectably Extended belt pouch thing. Explanation to come.) He's thinking about the upcoming trial, knowing that Greyback will go straight to the Veil after Daphne Greengrass gives her testimony. He knows the rough outline of what happened to her, and that, on top of everything else Greyback has done, will be enough to send him through the Veil a hundred times over. He tries to think of what he knows about Daphne: the cold beauty of Hogwarts, Slytherin, her family was attacked before their seventh year for refusing to declare for Voldemort (though they also weren't light side), and she didn't come back to school after that, etc.
The trial starts, people start coming in, everyone's either sucking up to him, which he hates, or demanding all kinds of answers on everything from him, which he also hates. Greyback is in the cage in the middle re: Barty Crouch Jr in the movies. Daphne comes in last, and he's shocked at how different and scarred she is from his (admittedly vague) memories of her. She wears high-collared formal robes and has her hair combed down over the side of her face, though some of the scars are still visible. After her testimony, they vote and he's found super guilty and they'll kill him that afternoon. Everyone else files out but Harry stays, pretending to shuffle papers, and just being super depressed (not about the trial, just about stuff).
That afternoon, all the people who want to/have to witness Greyback's death gather at the courtroom. These include: Harry, various Ministry officials, Daphne Greengrass and her father, Andromeda Tonks, Kingsly Shakelbolt, Mafalda Hopkirk, Percy, and a bunch of guards to hold Greyback. As they are going through another room to get to the Death Room, Greyback breaks free of his restraints and goes for Daphne, seeing her testimony as the last nail in his coffin. Harry grabs her out of the way and they both stumble into another area and fall into a vat of Time Sand, and all of the newly constructed Time Turners tumble down around them.
The next thing Harry is aware of is that he has no sense of smell, no sense of up and down, and he and Daphne are positioned back to back, naked, in the middle of a mass of swirling sand like a gentle tornado. They're both watching their own, and each other's lives from birth to the moment they fall into the Time Sand. Also, the mundane forms of the Hallows are floating around them as well, so a stick, a stone, and a plain grey cloak. It's revealed later, when they get back to the real world, that their powers have become enmeshed with Harry's magic and have also pushed the Horcrux out of him. Anyway, this is where readers get some background on what Harry's been up to since the end of the War, and what Daphne's life has been like in this universe generally. PLEASE STAND BY FOR MUCH TOO MUCH EXPOSITION.
Harry's backstory: Canon until right after the battle. He barely gets a break before Kingsly asks for his help doing everything because the public wants to follow Harry. They draw up a game plan for getting Hogwarts and the Ministry up and running again and catching all the Death Eaters who escaped. Kingsly offers him an emergency Auror position, which Harry tells him to also offer to Ron and Hermione, but it turns out they're only staying long enough for the funerals before going to Australia to find her parents. Harry helps arrange several funerals himself, including Lupin's, Snape's and Colin Creevey's. During this he's trying to rekindle things with Ginny, and they're formally back together about a week after the battle and three days or so before Fred's funeral. During the funeral, Harry has to leave before it's over because of a Death Eater sighting. Ginny forgives him, but it still stung a lot. Hogwarts is his home base while it's being rebuilt, where he was also taking care of clearing Snape's name and figuring out what to do with his stuff. Ginny is at the Burrow. Two weeks later, Ron and Hermione leave for Australia, but he doesn't even get a full goodbye because he's called away again. It feels weird for the threesome to be split up for the first time since pretty much ever. At some point in here, he has to go to the bank and make a formal apology to the goblins, who take half of his inheritance and forgive him. About a month after that, they're starting to get the Death Eater trials going, so that takes up a lot of time. The average day for him looks like, trial stuff all morning, then more often than not he's called away to help chase somebody, then he tries to go eat dinner with the Weasleys before going back to Hogwarts to help rebuild till be basically passes out. Not having a speck of free time anywhere means he doesn't have to feel the sad feelings, see? After nearly five months they've caught everyone except Greyback, so all that's left to do is a TON of trial stuff because Harry's a witness for so many of them (he helps get the Malfoys cleared by citing Draco and Narcissa each helping him out, though maybe Mr Malfoy has to serve a little time or pay a big fine or something, idk) and trying to get Hogwarts finished in time for September. PS, for executions, he fought for the Curtain, not the Kiss (so that's the Veil in the Death Room rather than the Dementor's Kiss, because it's A) way more fucking humane, and B) who really trusts the Dementors anymore anyway?). Meanwhile, Ron and Hermione are in Australia, having found her parents and are working with local healers to reverse the memory spells. That year on his birthday (he takes it off, good for him) he spends it with Ginny, celebrating the together time. But the very next day he's off chasing Greyback, so, so much for relaxing. Ron and Hermione come home at the 15-ish of August with Mr and Mrs Granger in tow and Ron joins the "emergency Auror department" and Hermione spends most of her time at Hogwarts. The two of them set up home in the Leaky Cauldron and are enjoying their space as a couple. Harry feels distant even from them because it's like he never even stopped fighting the war, it just changed forms, whereas they got some distance. Hogwarts is back together four days before classes start, and the last thing they do is erect a monument to all those who died in the Battle. It's near Dumbledore's grave, and there's a big celebration/memorial. Ginny and Hermione go back to school and Ron takes a flat in Hogsmeade and plans to do the Auror training when things calm down. Harry moves into Grimmauld place with Kreacher and Winky. Harry sees Ginny and Hermione off at Platform 9 ¾ with Ron. They're still chasing Death Eaters, though it's more and more rare that he has to go out these days. The trials are starting to take up more and more of his time, and Ron is already in Auror training. That Sunday, Harry goes to visit Ginny at school and she gushes about it being so therapeutic to be back in school and it feels like the war is finally over and Harry feels he's still in the thick of it, so there's a disconnect there. He takes up cleaning Grimmauld place because the nightmares make him hate sleeping. He eats at the Borrow a fair amount, but it's sort of awkward because Ginny's not there and Ron's usually not there, and George only wants to hear about Harry catching Death Eaters and the trials and punishments and stuff. Nothing really changes till Halloween, when Harry gets Ginny permission to get out of school to visit his parents' grave. He gets it, but this cements in her mind that they don't view their relationship the same way: Harry thinks she's his 'rock' and she just wants to put the war behind her and he won't let her. She decides she may have to break up with him, and he simultaneously decides to propose to her. So life goes on for a month or so, and Harry gets a ring. Harry skips seeing her once to get her the ring, and then again to go chase Greyback. So then Ginny has a Hogsmeade weekend, and Harry meets her there and makes a nice big picnic and she thinks maybe it can work out after all, but when he proposes to her, she's shocked and says she can't marry him right then, and he Apparates away and breaks contact with her. So things are terrible for a while, where she tries to write him letters and he doesn't read them. But Ron and George eventually talk him into going to the Christmas party at the Burrow, so he's on his way there, maybe stops at his parents' grave or something, when he sees Greyback (somehow), and goes after him and single-handed takes him down with only a small wound on his shoulder (nails, not teeth). Ron eventually catches up with him at St. Mungo's and is mad at him for skipping out on the family, but Harry has done the one thing he can't be mad about. Ron brings the whole family, but the nurses shoo them all out, so Ginny, who was planning to try and reconcile with him, doesn't get to. So she keeps sending him letters, which he probably reads, but just can't get past how hurt he was. Meanwhile, the Ministry is collecting grievances against Greyback midway through January, and on February 8th, the Greengrasses send their story about Daphne in from France, though they don't come home yet. So meanwhile there's a TON of administrative nonsense to be gotten through, and his trial isn't until late March. Harry is kept busy by Kingsly and various official things now that most of the trials and rebuilding are over. He's starting to take the Auror training and is seeing Teddy regularly on Saturdays and cleaning out Grimmauld place because he just can't seem to not be doing 30 things at once. In early April, he's released form Emergency Auror duties and is encouraged to focus on his training. The trial is only Monday through Wednesday, his training is Thursday and Friday, and he has Teddy on the weekends. Grimmauld place is looking good. But Ron is starting to get worried about him because he's so withdrawn and 'busy' all the time. The trial for Greyback starts with the most minor offenses and works up to killings, turnings and maimings, so Bill is near the end, several weeks later. Harry has to preside over the whole thing (note: Mr. Greengrass is also there for the whole thing because Greyback burned their house down so he testified in the beginning). Anyway, May 2 rolls around, and there's a big to-do planned at Hogwarts, and they want Harry to speak, of course. At some point shortly before this while he's running around happily ignoring his own problems in favour of helping plan everything (maybe coming back from visiting George and ordering fireworks for the party), he overhears Ron and Kingsly talking about him, where Ron is begging him to let Harry have time off and Kingsly thinks he's fine. Anyway, the party happens, and he can't avoid Ginny there, so that's a big confrontation and they kind of talk it all out and aren't together anymore, and not on good or bad terms. Ginny tells him all about how she has managed to move on, and he's not letting himself, and Harry goes away admitting that he hadn't paid enough attention to her. She says she'd like to try again once he's 'grieved' and he can think 'yeah, you were supposed to help me through that', so she's a bit bitchy and he's a bit selfish, fair fair. Meanwhile, Mrs. Greengrass and Astoria come back and begin rebuilding their manor. He has a dinner night with Ron and Hermione because Ron thinks he's going round the bend. He won't talk about Ginny, but otherwise carries the conversation off, despite the awkwardness of his two best friends dating. So the maiming, turning and killing testimonies happen, and then Daphne comes back (she hadn't before because she was so scared to face everyone). Her testimony is on June 24, and the verdict is revealed the next day, and he is sentenced to death by Curtain to be carried out immediately. Aaand that's where we catch up with the start of the story. You can use as much or as little of this as you like, I know we went a little over-detailed in some places, but that's the gist of what we're thinking.
NOW DAPHNE GETS WAY TOO MUCH EXPOSITION! Daphne's back story: born on August 5 1980. She's the second child of Mr. Ryker and Mrs. Luvinia Greengrass, born two years after her older brother, Blaine Ryker Greengrass, and a year before her sister Astoria. Her family is loving and warm, though very formal and she has manners training from a young age. They were totally neutral in the first War, though Ryker's cousin was a Death Eater. Nothing fell on them from either side, and the cousin was able to hide for years until someone fingered him and was flushed out by Aurors. When Daphne is 8, she and her family go to Hogsmeade because Ryker's cousin has been harassing them for help and shelter and Ryker wants to talk to his uncle, his cousin's dad about what to do about him. Mrs. Greengrass is just taking the kids shopping in the mean time. Little do they know, the cousin followed them with the intention of kidnapping Blaine, but he and Mr. Greengrass got into a confrontation and he wound up just shooting a Killer Curse and doing for Blaine. Woops. He was caught by Kingsly at that point, who was off duty and new. After that the family really changes: the parents kind of 'withdraw into their hearts' and have a hard time dealing with their son's demise. Daphne is suddenly the heir because Astoria's birth had complications and Mrs. Greengrass can't have more kids. So her fate, now, is to remain single and adopt one of the sons her sister will hopefully have, and if not, then have a lovechild. But she can't take anyone's name, which in their terms means no marriage. So she begins to cut herself off from the idea of intimacy and attachment, though she can't help but realize how attractive she is during Hogwarts, so she becomes very vain (Physical description: Ashy-silver blond hair, hazel eyes which are more green but brown-y/orang-y closer to the pupil. In years like, 3 or 4 through 6, she is the 'beauty of Hogwarts' and wears her hair long and out of her face with a slight curl to frame it. She is tall and nearly hour-glassy, medium bosom, small waist, some hips, the whole thing. She's stunning, and knows it). This leads to her 'ice queen' persona, since she refuses to get close to anyone because she knows nothing will be allowed to come of it down the line. She is in Slytherin, but she doesn't hold Muggle-borns in contempt for their lineage because she views them as Squib lines which have recovered their magic. She does sort of resent that they don't try hard enough to understand magical culture and instead try to bring their own culture to the wizarding world though. By the age of nine, she's being trained as the new heir, and is told the history of their family, which is when she learns that her great-grandfather was adopted from a Squib line from a few generations back where the magic reappeared, and since he still had the name, he was adopted and made the heir with no problem, and people thought he was his adoptive mother's lovechild. In Hogwarts, her best friend was Tracey Davis, though she keeps even her at an arm's length, and she followed Pansy because Pansy is queen of the heap and is dating Draco and that's just now the social scene works. Daphne is attracted to Draco, though she doesn't agree with his politics, she thinks that he might be a possibility for her babydaddy if Astoria doesn't have kids. (Oh, the irony.) In the summer after her sixth year, Greyback approaches the family looking to marry her, which they firmly (and rudely) refuse, and not just because she can't marry anyone, but because he's a freaky Death Eater werewolf (this is sometime in July, btw). So then, on August 1 (the day of Bill and Fleur's wedding), Greyback is coming from there, all crazed with bloodlust, and maims Daphne in the logic that if he can't marry her, no one can, his parting shot being "Who'll want her now?". This happens in their home, which they immediately vacate, only for Greyback to set it on fire anyway. They get her to St. Mungo's, but only for rudimentary help, and then go straight off to France, where they own a vacation home. Daphne recovers slowly, and her scars are really quite horrendous, which scares her family, thus alienating her pretty thoroughly. (After the attack, she has four long parallel scars running from the left side of her scalp, across her face and neck, crossing her breast to run across her stomach down to her opposite hip. Her eye is mangled and she can't open it, and she wears high collars and her hair hangs over her face from then on.) Instead of screaming and raging, Daphne basically cuts herself off from everyone because she views her situation as having the one thing she considered valuable about herself, her physical beauty, stripped away in a heartbeat. She sees her future as never being in public again and doing all her business through proxy. They hear about Voldy going down, and Mr Greengrass goes back to retake his Wizengamot seat, where he strongly supported the Curtain not Kiss policy of Harry's, but knowing that Greyback is on the loose keeps Daphne, Astoria and Mrs. Greengrass in France. Once Greyback is caught, Astoria and Mrs. Greengrass go back and start rebuilding their home, however Daphne stays because she's so ashamed of her scars. She eventually had to go back to establish her complaint against him, which is when Greyback is condemned, and she and Harry are thrown into the Time Realm.
Alright! With all that out of the way, we can move on! There is no sense of time in the Time Realm, ironically/appropriately enough, so they each watch both their own lives and each others' lives several times over. They eventually start to talk somewhat, though at a certain point, why bother when you've seen someone's whole life literally flash before your eyes? It's a level of intimacy they've never had before, though Harry had a taste during his Occlumency lessons, and again in the Pencieve. I don't think this is the time when they work out their Big Traumas necessarily, but maybe a Medium Trauma or two, just to establish a bond. Maybe he works through his attachment to Ginny a little and sort of lets that relationship go. They can't get out of the Time Sand tornado, not that they have many good options for how to try, but they can move around a little, and Harry does the gentlemanly thing and gives her the cloak that used to be the Invisibility Cloak so she can cover herself (and more importantly to her, her scars).
So the thing that gets them out of all this that Harry notices that at the exact same moment that Daphne's brother Blaine is being murdered in Hogsmeade, lil 8-year-old Harry is running away from Dudley and his gang of friends and Apparates to the roof of the school. (I'm like 90% sure this really happened in the books? At the very least, it's ~implied~ and that's good enough for me.) Anyway, he mentions this to Daphne, who says something about how both of their lives would have been so much better, or at the very least very different, if Harry had Apparated to Hogsmeade instead and interrupted her uncle killing Blaine. Harry reaches out to his 8-year-old memory-self and touches the memory, and suddenly is sucked out of the time realm back to reality where his soul and magical core merge with his child self's. He immediately changes the past by sort of accidentally changing the Apparition destination to Hogsmeade, where he practically mows Blaine over, since he was running before Apparating, so the curse goes over their heads and hits a random pigeon or something. All of this is extremely chaotic, since for all intents and purposes he's a 19 year old suddenly jammed into his 8 year old self's body, he's got no idea if Daphne's with him or not, and his magic is going BONKERS because of the merging of his magical cores and also the Hallows' powers are playing havoc with him too, so he's on the edge of unconscious when Remus, who's working at the Hog's Head for the moment, sprints over and recognizes him (meanwhile, Uncle Greengrass is being subdued by Kinglsy, who is a new, off-duty Auror, and this helps him make his name as Top Badass, so that's cool). Harry sees Remus and murmurs "Moon?" before passing the hell out.
While he's unconscious, Dumbledore arrives on the scene and whisks Harry off to the Hospital Wing while the Greengrasses hustle off to St. Mungo's to have Blaine checked over, and also Daphne, who seems to have "fainted from the shock of it all, poor dear thing". Harry is taken back to the Dursley's the next day, so he still has no clue if Daphne's with him or what, and who he can contact in the magical world or how. But he's soon got bigger fish to fry, since his accidental magic is off the freaking charts right now, and this is also probably a good time to discuss exactly what powers he got from the Hallows. The Cloak gives him the power to become invisible on his own; the Wand just makes him way over powered, though that's a problem when he's learning new spells and stuff because they can go really dramatically wrong if he doesn't have them perfected; the Ring is less of a cool superpower than a sort of confusing, troublesome thing he sort of wishes he couldn't do. Basically if he's there when someone dies, he sees their soul departing into the next world and be able to hear things from the other side. After he sees someone die for the first time he wonders if he can open the veil on his own and tries it out. He can only hold it stable for a few seconds and it is INTENSELY draining, so it's a lot of risk and exhaustion for not a lot of reward, but it's a thing he can do; then also, not a power so much as just a thing, is when he gets out of the time realm he starts having dreams about the Hallows' creation, with Eurydice Peverell and her brothers arguing about how to do it and if to do it and then doing it and it all going wrong and how they live out the rest of their lives. More on that in a sec. So clearly he doesn't figure any of this out right away: as far as he's concerned his magic is acting up because he has a fully developed wizard's power crammed into an inexperienced child's body, but that doesn't mean it's not FREAKING ANNOYING.
Meanwhile (we had envisioned this as being only from Harry's POV, but if you want you can include Daphne/Dumbledore/other viewpoints if you don't feel like explaining all this other stuff later, up to you), Daphne doesn't give even half a shit about not changing the past (and it's too late anyway since they already saved Blaine) so she takes matters into her own hands and sends an anonymous note to Rita about Harry's living conditions, and if you don't think she got her little winged butt over to Little Whinging faster than you can say Bertie Bott's Every Flavoured Beans you've got another thing coming. So she scouts around the place, sees Harry trying to control his magic with not very great success, sees the Dursleys being the Dursleys about it, and is just over the moon. If you want, you can make it so that this is the story that makes her name in the wizarding press. Anyway, Harry doesn't notice her till it's too late and she's already leaving, lol, oops. So he's sure there's going to be some kind of firestorm in the Ministry/the wizarding world in general, but is also sure there's nothing he can do about it so he just focuses on trying to control his damn magic.
There is, in fact, a firestorm when Rita's article comes out a few days later. The Boy Who Lived, being abused by Muggles?! UNACCEPTABLE. It comes out that no one ever actually read the Potters' will as regards to custody, so that gets brought out, and it basically says, "In the event of our death and/or incapacitation, Harry goes to Sirius. If not to Sirius, to Remus. If not to Remus, to Frank and Alice Longbottom. He does not go to Petunia and Vernon under any circumstances." Sooo…. That's awkward for Dumbledore. It's decided that he can't stay with the Dursleys, and that according to the will he has to stay with Remus, which raises the question of whether this is a good time for Remus' lycanthropy to come out or not. Either way, Harry should still go to live with him, all it changes is public opinion. Anyway, Dumbledore, who has been feeling as though he's lost control of the whole situation, which in his book is bad, takes Remus under his wing and promises to take care of everything: he'll ensure they have an income, and work, and a place to live. That place to live, you ask? The Shrieking Shack. I enjoy that notion intensely, so that's what we're going with. It's gonna get all fixed up and fancified (and warded to HELL) before they move in, but it's still the Shack. It's privately arranged that he'll stay with the Greengrasses during the full moon, since Blaine owning Harry a life debt sort of gives them some rights to him and they could have made a good argument for custody if the Potters' will hadn't been so explicit.
So anyway! Harry actually becomes aware of all this when Moody, Kingsley, Madam Bones, and Dumbledore all show up at Privet Drive and take him away from a seriously frazzled Petunia and Vernon. Harry probably has some accidental magic right then and there when he sees them all, just from being shocked out of concentrating on controlling it, so that can be funny. And he's also not supposed to know who any of those people are, so mitigating his reaction to them can be a fun time too. And speaking of accidental magic, you've all been so extremely patient so far that now I will in fact tell you what our idea for the Hallows' history is. Hang onto your pants, we're getting weird!
So the history of the Peverells and the Hallows is as follows: there are still the three brothers, but Death as a sentient entity was a later addition to the myth and didn't really happen. The things the storybook version missed out on was that they actually had a sister named Eurydice, and that she was the mastermind and ringleader of the Hallows' creation. She wanted to become Master of Death and invented this whole ritual which would use the power of her own death to imbue each of the Hallows with the abilities she intends. She can then be resurrected by their power and live forever. She forces her brothers to help her with the ritual, but something goes wrong and she's not resurrected, just dead. The Hallows work as they should, but they can't bring her back no matter what they try. So each brother takes one Hallow and they all go their separate ways, haunted by grief and shame. The story got their lives thereafter pretty much right: Antioch, the oldest, becomes reckless and mad and eventually provokes a man into killing him; Cadmus, the middle, committed suicide (but maybe came back as a ghost just for extra irony); Ignotus, the youngest, ran from his guilt all his life till dying at an advanced age. Cadmus has a son at one point, who still leads to the Gaunt family, and Ignotus' son leads to the Potters, so the Ring and the Cloak still get passed down as in canon. (Honestly I think this idea by itself would be a really cool fic so if you just wanna grab this piece out and run with it you have my complete blessing.) So when Harry collects all of them in the seventh book, he is as much the Master of Death as anyone knows it is possible to be, and it's true that the confluence of the Hallows leads to some weird magical effects on the person holding them. But now that the powers of the Hallows have become meshed with Harry's magic after their trip through the Time Realm, Harry LITERALLY IS THE MASTER OF DEATH. His powers are enumerated above, but the other weird thing that starts happening is that he gets dream flash-backs of Eurydice and her brothers, first arguing over if they should do it (make the Hallows), then how they should do it, then doing it, and the aftermath. So that's a weird mystery that he doesn't really solve till close to the end, but we're not there yet. As for how he has the Ring in the first place, the best thing I can think of is to have him go back to the place of his 'death' during the dedication/memorial at Hogwarts on the first anniversary of the Battle, and happens to see the Ring in the brush. He gets worried (somewhat unreasonably so, but he's not at his best at that point) that someone like George might find it and end up like Cadmus did in the story version, so he picks it back up and carries it around till he can figure out what the hell to do with it. So that's what's up with the Hallows. Back to the story.
Once he's away from the Dursleys and has had the situation explained to him, he's super eager to see Daphne because he hopes she came through with him and he'll have an ally in this whole crazy situation. His visit with them is pretty well supervised—they're no great favorites of Dumbledore's since they didn't take sides in the War and were related to a Death Eater who escaped capture for hella long, and Remus is pretty protective of him just because he has the details of what the Dursleys were like and now wants Harry to have only the best experiences ever. Also, Blaine is super excited to meet the kid who saved his life, so he monopolizes Harry for most of it, but Daphne sidles up and whispers "Greyback" or something in his ear, so he knows she's there the same way he is. Woo!
So there's a period of minor chaos while the Shack gets fixed up enough for them to live in, they move in, but then the full moon happens like really soon after that so he goes right over to the Greengrass' for a couple nights (whether they know about Remus or not is up to you—there's a general prejudice against lycanthropy in the wizarding world, and though the Greengrasses aren't at all into the pureblood supremacy ideology, they are a pureblood family with all the baggage that comes with that.) This is when he and Daphne get to talk properly for the first time. Harry spends a minute freaking out about how they're changing the past, which is the biggest no-no of time travel there is, but Daphne's like "lol, well, it's a little late to worry about that considering MY BROTHER IS ALIVE" which pretty well kills that argument. They identify three major things they have to deal with. A) Get Sirius out of Azkaban; B) destroy all the Horcruxes; C) kill Voldemort for real. There are a couple little problems. Mainly, they're 8. Not only does that make wandering around solo highly suspicious, but they still have their Traces on them, so larking off to non-magical areas like Little Hangleton is going to look highly weird to whoever's in charge of monitoring Traces. They tentatively say that Harry can chalk any weird stuff he does up to accidental magic for at least a little longer, but still. Nextly, the easiest way of exonerating Sirius is to expose Pettigrew, but neither of them have a logical way of getting close to Ron/the general Weasleys. The Greengrasses don't socialize with them and Remus was never close with Molly or Arthur, though he might have known Molly's brothers since they were in the first Order of the Phoenix before they died. Then lastly, some of those Horcruxes are going to be harder to deal with than others: the Cup is already in Bellatrix's vault, Nagini may not exist yet, Harry is Harry (they don't yet know the Hallows' power pushed the Horcrux out of him), and they'll need Sirius before they can get into the Black family house to get the locket.
So, all that established, we've got a couple years to kill before they start Hogwarts. You can deal with this either by a time skip or by describing how they try to deal with some of those hurdles, like trying to make friends with Ron and asking to go on weird vacations to obscure places to try and lay hands on some of the Horcruxes already, but all that is up to you.
Once they get to Hogwarts, the Sorting is surprisingly stressful. It hadn't even crossed the farthest corners of their minds that it would be an issue, but the Hat is just BEYOND interested in them and spends a hella long time talking to Daphne about how they should be sorted and what would be the most beneficial for their goals, so she ends up being a practically record-setting Hat-stall. They end up going to the same Houses they were in originally, because Daphne thinks it'll really help inter-House unity later on and also if it ain't broke don't fix it, so by the time Harry gets up to the Hat it's just like "We figured it out. You're going to Gryffindor. Come steal me out of the office sometime and we'll talk." and Harry's just like "Uuh, cool." (by the by, Blaine is in Hufflepuff, going into third year when our two start Hogwarts.)
First year: mainly just Harry learning to reign in his own magic, though he had made a great start on this in the last couple of years, but it turns out that having a wand really helps, some mumbo jumbo about magical focal points and whatnot. Meanwhile Daphne, who's not doing the Slytherin cunning thing much credit, is sort of rampaging around having a great time fucking up the past, because the past fucking sucked. The character dynamics between them are actually pretty interesting: on the one hand, Daphne feels like she has a new lease on life. After going through Hogwarts the first time as the proverbial ice queen, now she can actually have fun and enjoy herself and make friends. Harry, on the other hand, who had a fairly normal, happy Hogwarts time (minus the Voldemort stuff, but you know), is now so determined to save everyone that he's not even taking time to enjoy himself. So when Daphne's like "Come on, we're only young once!" not only is that nice and ironic (can you tell I like irony yet?), it's also a complete 180 from their original characterizations. Anyway, they grab the diadem out of the Room of Requirement like, their first night in the castle, and they're completely ignoring Quirrel, since they knows Voldemort can't get the Stone out of the Mirror, which is confusing as fuck to Dumbledore. The big thing they succeed at is getting Pettigrew caught, by Harry Imperioing him to go around as a human and get seen/caught by a professor. If he is caught, he still gets away, but enough people saw him that there's no question as to who he was and what it means. So that sets the getting-Sirius-out-of-Azkaban thing in motion, yay! But at the same rate, he can't get the Chamber open. He can't speak Parseltongue. WHY CAN'T HE TALK TO SNAKES? This throws a wrench in the works because they were counting on having the Basilisk fangs handy to kill the Horcruxes as they collect them, and now they'll have to figure something else out, and also worry about the damned thing running rampant in second year, which is going to be a huge problem if Harry can't hear it. (funny note: Slytherin still gets the House Cup at the end because Harry has no derring do in the dungeons for Dumbledore to reward, hahahaha)
Second year: Yay, Sirius is out! Harry works out some excuse to go to Grimmauld Place and nabs the locket. But still, he can't talk to snakes? What does that mean about his being a Horcrux? Is there a way to test for that without killing him? And also why isn't Dobby fucking his life up? Just because he lives in the magical world now? Okay…. But now why isn't Mr Malfoy giving Ginny the diary?! He and Daphne are still trying to get into the Chamber, without any success whatsoever. They end up having to get chummy with Darco, especially Daphne, so that she can get an invite to the Manor during Easter break or something and snag the diary that way. Daphne gets super sick of Lockheart halfway through the year and sends all his cover-ups to the Prophet so he skips town at like Christmas. Remus takes over for the second half of the year, yay!
Third year: Before the year starts, they make an excursion to Little Hangleton, maybe by Portkey so they don't get picked up for underage Apparation or something, and try to get at the Ring. This might take a couple of visits to get right, but the weird thing is that it's like the Ring is also trying to get free of the enchantments to get to Harry. It recognizes that Harry has the same sorts of powers that it does, as well as the echo of its creator, Eurydice, and wants to be with him. Harry doesn't know this, but Dumbledore has also been having trouble with the Elder Wand and can't figure out freaking why. But so they get the Ring! The big challenge they're taking on next is getting the Cup out of Gringotts. They go through all kinds of weird plan ideas and whatever they eventually settle on, they don't destroy the whole bank, and may just end up Fiend Fyering Bellatrix's whole vault with all the rest of the Horcruxes they've collected. Meanwhile, Sirius has been teaching Defense all year, but decides at the end of the year that he'd rather go into Auror training than just teach the stuff, so the curse still works even though no one dies or anything.
Fourth year: the Triwizard Tournament is still a thing because Voldy still needs blood of his enemy. Just after the champion-choosing is when he and Daphne get drunk somewhere together and commiserate on about how no one's going to understand them because of the time travel (maybe one of them is getting out of a casual relationship or something) and end up making out all night. So they're sorta together now, yay! In my mind that's a very low-key element of the story, since romance is only a small part of their larger relationship at this point, but it's another way they can rely on each other and I think that's very nice. Anyway, the tasks: Harry (and consequently the other champions too) gets a different dragon and manages to make it nap hella hard, so hard that it doesn't wake up for ages and the other contestants have to be postponed till she's dragged out of the way; they take Daphne for the second task, duh. Harry uses a combo of the Bubble-head charm and Muggle flippers and stuff so he's super fast and has practiced in the lake a lot so he knows exactly where to go, and he just brings Daphne up rather than waiting around and also bringing Gabrielle because he knows she'll be fine. Third task: Harry has a huge points lead so he goes into the maze way before anyone else, and is also just way more magically prepared so he gets there wicked easy. Also, they save Crouch Sr before he dies, and give him Draught of Living Death so that he doesn't fuck up Harry getting to the graveyard. They set Winky to take care of him during the task and give her the antidote with instructions to give it to him just after the third task ends, but she jumps the gun a little and gives it to him during the task instead so he comes storming down and unmasks Crouch Jr and the whole truth comes out about his plan so Dumbledore makes an announcement across the whole maze that no one should touch the Cup, but Harry ignores him cause he has to go kill Voldemort, haha. They get the blood from him, but since he's low-key willing to give it Voldemort comes out weaker than canon. Harry assumes it's because all the Horcruxes are dead. They duel, Harry's primary objective being to kill Nagini, which he does do, which enrages Voldemort and causes him to throw off a Killer Curse at Harry, which he gladly accepts. So he goes to a place like the misty place of canon, meets Eurydice for real as she explains (briefly) that he's the Master of Death and will live out his natural span. Since he wasn't a Horcrux it turns out that he really truly would have died if he hadn't had the Hallows' power inside him, so that was lucky. He wakes back up after her spirit dissipates, finally passing properly into the next world, but Voldemort stays dead and all the Death Eaters freak out and leave quick-smart, so Harry just Portkeys back to Hogwarts where everyone's FREAKING OUT. The end :)
Epilogue can be on the 'second' date of the day they originally time traveled, June 25 1990, only this time they're together and happy and maybe have a kid even though they're only 19 and as the clock chimes midnight Harry lets go of the fear that they'd be somehow stuck in a time loop and all the things they'd worked for would be ripped away again.