Reminders

$schedule = new ReminderSchedule(1440, 60);   // a day before, then an hour before
$schedule->moments($booking->interval);        // the two moments, the earliest first
$schedule->due($booking->interval, $now, $lastSent);

The offsets are minutes before the start, from 1 to a year, at most 20; they are kept once each, the furthest first. An offset out of bounds, or more than 20, throws InvalidDefinition.

due() gives the offset of the reminder to send now, or null: the one closest to the start whose moment has come, after the last one sent, as long as the booking has not started. A reminder whose moment passed unsent is overtaken: a booking made an hour before its start gets the hour's reminder only, and a job that ran late sends one reminder, not several. The application keeps the offset it sent last and passes it back; the library keeps nothing.