Your Timehop Day is a 24 hour period between 5am and 5am.
Your Timehop updates at 5am each day, because wouldn't it be strange to cut a day in half right at midnight? You'd see your 1am posts from New Year's Eve AFTER New Year's Eve. Ain't nobody got time for that!
5am feels like the a safe bet that your "day" is fully concluded*.
*But, if you're out partying past 5am on the reg, we salute you.