integer representing day of month.
(<=> (Day-Number ?Day-Of-Month)
(And (Integer ?Day-Of-Month)
(=< 0 ?Day-Of-Month)
(=< ?Day-Of-Month 31)))
(=< ?Day-Of-Month 31) (=< 0 ?Day-Of-Month) (Integer ?Day-Of-Month)
(<=> (Day-Number ?Day-Of-Month)
(And (Integer ?Day-Of-Month)
(=< 0 ?Day-Of-Month)
(=< ?Day-Of-Month 31)))
(=> (Timepoint.Day $X $Y) (Day-Number $Y))
(=> (Ref.Day $X $Y) (Day-Number $Y))