(** 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 [@@deriving sexp]

(** [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

(** ISO-8601 rendering and validating parsing. [of_iso8601] enforces the same
    domain as {!make}. *)
val to_iso8601 : t -> string
val of_iso8601 : string -> (t, string) result

val weekday_to_string : weekday -> string

(** Sexp form is an ISO-8601 atom, e.g. [2026-04-05]. [t_of_sexp] validates and
    raises [Of_sexp_error] on a malformed or out-of-domain date; loaders convert
    that to a [result]. *)
val t_of_sexp : Sexplib0.Sexp.t -> t
val sexp_of_t : t -> Sexplib0.Sexp.t