3 files changed, 42 insertions, 3 deletions
diff --git a/lib/kernel/date.ml b/lib/kernel/date.ml
index 525b35b..cf5aabd 100644
--- a/lib/kernel/date.ml
+++ b/lib/kernel/date.ml
@@ -1,4 +1,4 @@
-type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat
+type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat [@@deriving sexp]
 
 (* Internal representation: rata die = days since 1970-01-01 (proleptic
    Gregorian). Howard Hinnant's civil<->days algorithm; OCaml's `/` truncates
@@ -56,3 +56,26 @@ let weekday t =
   (* rata 0 = 1970-01-01 = Thursday (index 4 with Sun=0). *)
   let w = (((t + 4) mod 7) + 7) mod 7 in
   [| Sun; Mon; Tue; Wed; Thu; Fri; Sat |].(w)
+
+let weekday_to_string = function
+  | Sun -> "sunday" | Mon -> "monday" | Tue -> "tuesday" | Wed -> "wednesday"
+  | Thu -> "thursday" | Fri -> "friday" | Sat -> "saturday"
+
+let to_iso8601 t = Printf.sprintf "%04d-%02d-%02d" (year t) (month t) (day t)
+
+let of_iso8601 s =
+  match String.split_on_char '-' s with
+  | [ y; m; d ]
+    when String.length y = 4 && String.length m = 2 && String.length d = 2 -> (
+      match (int_of_string_opt y, int_of_string_opt m, int_of_string_opt d) with
+      | Some year, Some month, Some day -> make ~year ~month ~day
+      | _ -> Error (Printf.sprintf "date %S: not an ISO-8601 date" s))
+  | _ -> Error (Printf.sprintf "date %S: expected YYYY-MM-DD" s)
+
+let sexp_of_t t = Sexplib0.Sexp_conv.sexp_of_string (to_iso8601 t)
+
+let t_of_sexp sexp =
+  let s = Sexplib0.Sexp_conv.string_of_sexp sexp in
+  match of_iso8601 s with
+  | Ok t -> t
+  | Error msg -> Sexplib0.Sexp_conv.of_sexp_error msg sexp
diff --git a/lib/kernel/date.mli b/lib/kernel/date.mli
index b684d61..2f632c8 100644
--- a/lib/kernel/date.mli
+++ b/lib/kernel/date.mli
@@ -3,7 +3,7 @@
     arithmetic and comparison are total and cheap. *)
 
 type t
-type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat
+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. *)
@@ -21,3 +21,16 @@ 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
diff --git a/lib/kernel/dune b/lib/kernel/dune
index b63c476..b909ac5 100644
--- a/lib/kernel/dune
+++ b/lib/kernel/dune
@@ -1,2 +1,5 @@
 (library
- (name colitur_kernel))
+ (name colitur_kernel)
+ (libraries sexplib)
+ (preprocess
+  (pps ppx_sexp_conv)))