Date plus days

Adding a fixed number of days is unambiguous arithmetic — no month lengths, no clamping. A hundred days from 1 January 2026 is 11 April 2026, a Saturday.

Fixed day counts turn up wherever a period must be unambiguous. Ninety-day payment terms, thirty-day returns windows and hundred-day political milestones are all day counts precisely because they cannot be argued about — a calendar month depends on which month, and a fixed count does not. The trade-off is that a day count drifts against the calendar, so a 90-day cycle never falls on the same date twice.