How to schedule one-off or repeating tasks
(TODO. Not yet implemented: a cron-like program will eventually respond to assertions demanding periodic or delayed execution of tasks (likely expressed as assertions, making it more of a delayed-or-periodic-assertion-producing program).)
Timer tolerances
Apple has come up with the useful idea of a timer tolerance, applicable to both repeating and one-off timers. In their documentation, they write:
The timer may fire at any time between its scheduled fire date and the scheduled fire date plus the tolerance. [...] A general rule, set the tolerance to at least 10% of the interval [...] Even a small amount of tolerance has significant positive impact on the power usage of your application.