fix: use event TZID for UNTIL date conversion

Add a `tz` field to `rem` to carry the event's DTSTART timezone, and
introduce `timedesc_of_utc_or_timestamp_tz` to convert UNTIL timestamps
in that timezone instead of the process locale. This makes UNTIL
comparisons locale-independent for events with a known TZID.

bin/utils.ml

@@ -112,6 +112,14 @@ let timedesc_of_utc_or_timestamp_local (ts : utc_or_timestamp_local) : Timedesc.
   (* this case is not present in my current dataset… *)
   | `Utc t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:local_tz
 
+(** Convert a UTC-or-local timestamp to a Timedesc.t in the given timezone. Use this (instead of
+    [timedesc_of_utc_or_timestamp_local]) when the event has a known TZID, so that UNTIL comparisons are independent of
+    the process locale. *)
+let timedesc_of_utc_or_timestamp_tz (tz : Timedesc.Time_zone.t) (ts : utc_or_timestamp_local) : Timedesc.t =
+  match ts with
+  | `Local t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:tz
+  | `Utc t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:tz
+
 let get_exdates ev =
   let event_props = ev.props in
   let dates_or_datetimes =