Date_sig.S.Period

`sig
type +'a p constraint 'a = [< Date_sig.S.field ]
type +'a period = 'a p constraint 'a = [< field ]
type t = Period.date_field period
val empty : [< Period.date_field ] period
val add : ([< Period.date_field ] as 'a) period -> 'a period -> 'a period
val sub : ([< Period.date_field ] as 'a) period -> 'a period -> 'a period
val opp : ([< Period.date_field ] as 'a) period -> 'a period
val equal :
[< Period.date_field ] period -> [< Period.date_field ] period -> bool
val compare :
[< Period.date_field ] period -> [< Period.date_field ] period -> int
val hash : [< Period.date_field ] period -> int
val make : int -> int -> int -> Date_sig.S.t
val lmake : ?year:int -> ?month:int -> ?day:int -> unit -> Date_sig.S.t
val year : int -> [< Date_sig.S.field > `Year ] period
val month : int -> [< Date_sig.S.field > `Month `Year ] period
val week : int -> [< Date_sig.S.field > `Day `Week ] period
val day : int -> [< Date_sig.S.field > `Day `Week ] period
exception Not_computable
val nb_days : [< Date_sig.S.field ] period -> int
val safe_nb_days : [< `Day | `Week ] period -> int
val ymd : [< Date_sig.S.field ] period -> int * int * int
end`