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))