(** Proleptic Gregorian calendar dates over the supported domain 1583..9999.
    [t] is opaque; the internal representation is a day-count (rata die) so
    arithmetic and comparison are total and cheap. *)

type t
type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat

(** [make ~year ~month ~day] validates the date; rejects a month/day out of
    range and any year outside 1583..9999. *)
val make : year:int -> month:int -> day:int -> (t, string) result

val year : t -> int
val month : t -> int
val day : t -> int
val weekday : t -> weekday

(** [to_rata]/[of_rata] expose the underlying day-count (days since 1970-01-01);
    [of_rata] and [add_days] are unbounded total arithmetic (they may denote a
    year outside 1583..9999 — only [make] enforces the domain). *)
val to_rata : t -> int
val of_rata : int -> t
val add_days : t -> int -> t
val compare : t -> t -> int