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.


  
