Effective dating: links that stop being true — Half-open periods

A period includes its from and excludes its to. A link running to 1 July and another starting 1 July do not both cover that day — the first has already ended when the second begins. This convention is applied in exactly one place in the product rather than restated at each call site, which is what stops half the comparisons ending up inclusive on both ends. The symptom of getting it wrong is a link that belongs to two periods for a single day, and it is precisely the kind of thing nobody notices until a total is out by one. You choose whether periods are measured by day or by instant when you turn dating on. Tom's link ends on 1 September and Priya's starts on 1 September, so on that day the account has exactly one owner — not two.