SPTP — Frequency

"2) Frequency. While the probabilistic definitions are included (and
necessary), there are many other instances of repeatable events that need to
be described e.g., Every Third Monday, the 15th of the month, twice a day.
(or a really hard one, every Easter). Though you may feel that these events
are out of scope, they are important in any sort of scheduler, and are
necessary to ensure that the low-level time definitions scale up. It would
not be good to use incompatible time/frequency expressions at different
scale."