Fixes T4325. We have a fair amount of (60 * 60 * 24 * 15) // 15 days stuff in the codebase, and at least one of them was wrong (15 hours, when 15 minutes was probably intended). I was about to write some more, so I wrote this instead.
Make these self-documenting and harder to get wrong.
(This function could be more sophisticated, but I think this is the only real use case we hit.)