`module Make (D : Date_sig.S) (T : Time_sig.S) : Calendar_sig.S with module Date = D and module Time = T`

Implement a calendar from a date implementation and a time implementation.

`module Make_Precise (D : Date_sig.S) (T : Time_sig.S) : Calendar_sig.S with module Date = D and module Time = T`

Similar to `Make` but results are more precise. The counterpart is that some operations are less efficient.