"rank at X time" based payouts are bad, especially for short time frames and small groups. It's almost always going to be an inconvenient time for a moderate percentage of your player base. It makes scheduling maintenance almost impossible for an internationally played game. It encourages "cheesy" gameplay like clock wars instead of playing the actual game.
Rewards should be based on the highest rank achieved during X time period, NOT the highest rank held at exactly X time. Look at Hearthstone, they pay out their arena ladder once a month based on the highest rank hit at any point that month. It's pretty universally liked, allows Blizzard to take servers down entirely at pre-scheduled times for proper maintenance without negatively effecting players, it allows players to play when they want, and it allows players who have reached their goal for the arena season to play fun decks instead of metasheeping 24/7.
