Time slips away like sand through cupped hands. They're constantly aware of it, aware of each coarse grain as it falls through the crevasses and settles onto the beach of used moments and breaths. It's hard to push it from their minds when there is so little else to do. They have their small family, but they have no other social contact. They have assigned tasks to do for the Order, but they can't throw themselves in to the real fray. They have games and toys and books, but they have nothing meaningful to do long-term. All they can do is occupy themselves until other people in other places decide that enough is enough and declare the fighting over.
The little things they amuse themselves with may give them contentment and temporarily distract them from the realities of the situation, but they are just that; distractions. And the pair knows that, ultimately, they're just distracting themselves and Harry and hoping that the war ends before their supply of sand depletes itself too far. What if they turn seventy before Voldemort is defeated and it's safe to take Harry outside again? What if they are forced to home-school Harry instead of sending him to the place that is like a second home to them?
What if their hourglass runs out before the war's does?