Lots more date rules

[MarcusRainbow]

Things like Modified Following, the third Friday in the month rule. Rules to step to the start or end of the current month. Rules to add a number of years or months.

One question is whether to implement a concise algebra of rules, designed to allow composition of these more ad-hoc rules. Or whether to just implement the ad-hoc rules.

This would be a nice self-contained issue for a new programmer.