I need to be able to limit the time units used in a duration. One example is the
duration of a pregnancy (or gestation) which is described in weeks and days only -
never months or hours. So it is necessary to restrict the duration characters to W
(for weeks) and D (for days) - where the possible are YMD(T)HMS - where T is the
separator for the time part of the duration. Restricting the time part will require
the THM - or for minutes will be TM.