Dumbledore went back in time exactly one year. He went to the first day of Harry's fifth year at Hogwarts using a special time turner. He saw Harry walking to the dorms with some friends and summoned him to his office.
"Harry, I know you want to yell at me about some things, but that must wait, I have something very important to tell you," Dumbledore explained.
Harry stared at him bewilderment, first Dumbledore ignores him all year, then he's suddenly sharing important information with him.
Dumbledore took a very deep breath, "in less than a year from now, you and all your friends will die."
Harry's mouth opened wide.
"Hogwarts will be destroyed and the world as we know it will end," he continued.
Harry's mouth opened wider.
Dumbledore stared right into Harry's eyes, "Voldemort will win Harry."
Harry's mouth opened as wide as it could.
"but there is a way to prevent it,"
"what?" questioned Harry.
"you must go back in time to when Tom Riddle was in Hogwarts, his sixth year. You must prevent Myrtle Morrison from dying," Dumbledore explained.
"what does moaning myrtle have to do with anything?" Harry questioned.
"everything!" Dumbledore yelled, "absolutely everything, you must trust me Harry, and if you do this right then Voldemort would never have risen."
Harry pondered this.
"your parents will be alive, but I doubt you'll be famous," Dumbledore enlightened.
"I don't care about that at all, I'm not famous in a good way anymore," Harry sighed.
"think long and hard about it Harry, there will many perils," Dumbledore warned, "you're going back to a very dangerous time at Hogwarts."
Harry thought about the ghost in the bathroom, then about the photographs of his parents holding him when his was merely an infant, "I'll do it."