Things to do: * Test with yhc. * Define ranges of times and dates. The set of all Mondays, for instance, or the set of all second weeks in the month. Any such functions would have to take a start date because the only alternative is to start with the epoch, and that would not be efficient. The existing date-time functions look a bit clunky for this job, but the proposals of Ashley Yakeley at http://semantic.org/TimeLib/ look more suitable.