Compute time expressions.
Example:
- the 2nd monday of April: Seq::nthof(2, Seq::weekday(1), Seq::month(4))
- 3rd week of June: Seq::nthof(3, Seq::from_grain(Grain::Week), Seq::month(6))
- last weekend of the year: Seq::lastof(1, Seq::weekend(), Seq::from_grain(Grain::Year))
- a sequence of all days monday 28th:
Seq::intersect(Seq::weekday(1),
Seq::nthof(28, Seq::from_grain(Grain::Day),
Seq::from_grain(Grain::Month)))