Dates, streaks and statistics

Every "today", "due today", streak, countdown and statistic in an Expo app is computed on the user's local civil-date keys from one injected clock, survives midnight, DST, travel and a clock set back, is folded by pure tested functions in one write per event, and reaches the screen as a finished summary on iOS and Android alike; two checkers prove it on the app's own code.

Rules that must hold