Module Period

module Period: sig .. end
A period represents the time passed between two events (a date, a time...). Only an interface defining arithmetic operations on periods is defined here. An implementation of this interface depends on the kind of an event (see module Time.Period, Date.Period and Calendar.Period).

type date_field = [ `Day | `Month | `Week | `Year ] 
module type S = sig .. end
Common interface for all periods.