Some date algorithms

[lpil]

https://howardhinnant.github.io/date_algorithms.html

Unclear if any of these will be useful here, but I wanted to make note of them. It may be that these are more suited to community package.

[dzuk-mutant]

I would personally find these useful (eg. the other day I was thinking of using this package, but I noticed I couldn't derive things like days of the week from a Timestamp so I tried gtempo instead) , but if they don't seem good for the core package I might try to make something myself at some point.

[lpil]

The expectation is that community packages provide further functionality for the time types provided. It may be that some functionality is promoted into this library, but supplementary libraries are the way to go for the immediate future.