ascending is identical to compare. descending x y = ascending y x. These are
intended to be mnemonic when used like List.sort ~cmp:ascending and List.sort
~cmp:descending, since they cause the list to be sorted in ascending or descending
order, respectively.
Returns the time-span separating the two of-days, ignoring the hour information, and
assuming that the of-days represent times that are within a half-hour of each other.
This is useful for comparing two ofdays in unknown time-zones.