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diff --git a/lib/kernel/date.mli b/lib/kernel/date.mli
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+(** 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