time-period-between expects datetime-provider?

[nick-chandoke]

The example expression (time-period-between (time 10 30) (time 11 00) '(seconds)) fails because seconds-between (and any other *-between functions) require datetime-provider?, as per their contracts. Thus time-period-between fails for all time-provider? that aren't datetime-provider?, regardless of fields. An easy fix is providing datetimes with dummy year, month, and date, e.g. (time-period-between (datetime 1 1 1 10 30) (datetime 1 1 1 11 00) '(seconds)), but at this point time-period-between is identical to period-between.