feat: add --timezone CLI option for configurable output timezone

bin/commandLine.ml

@@ -5,12 +5,16 @@ let files =
   let doc = "Files to process" in
   Arg.(non_empty & pos_all string [] & info [] ~docv:"FILE" ~doc)
 
+let timezone =
+  let doc = "Target timezone for output (e.g. Europe/Rome). Defaults to local timezone." in
+  Arg.(value & opt (some string) None & info [ "timezone"; "z" ] ~docv:"TZ" ~doc)
+
 let main_command f =
   let doc = "Convert iCalendar files to remind format" in
   let man = [] in
   Cmd.make (Cmd.info "ical2rem" ~version:"%%VERSION%%" ~doc ~man)
   @@
-  let+ files = files in
+  let+ files = files and+ tz = timezone in
-  f files
+  f tz files
 
 let main f = Cmd.eval @@ main_command f

bin/main.ml

@@ -14,7 +14,8 @@ let read_file filename =
   close_in ic;
   Bytes.unsafe_to_string s
 
-let ical2rem ical_files =
+let ical2rem tz_opt ical_files =
+  Utils.init_target_tz tz_opt;
   let good_rems =
     ListLabels.fold_left ~init:[] ical_files ~f:(fun good_rems_acc filename ->
         try

bin/utils.ml

@@ -1,5 +1,14 @@
 open Icalendar
 
+(** Target timezone for all timestamp conversions. Defaults to local timezone; overridden by --timezone CLI option
+    before any processing begins. *)
+let target_tz : Timedesc.Time_zone.t ref = ref Timedesc.Time_zone.utc
+
+let init_target_tz (tz_opt : string option) : unit =
+  match tz_opt with
+  | None -> target_tz := Timedesc.Time_zone.local_exn ()
+  | Some name -> target_tz := Timedesc.Time_zone.make_exn name
+
 type months = Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec
 
 let timedesc_wd_to_ical (wd : Timedesc.weekday) : Icalendar.weekday =
@@ -89,28 +98,28 @@ let string_of_span (sp : Timedesc.Span.t) : string =
   spf "%02d:%02d" hours minutes
 
 let timedesc_of_timestamp (ts : timestamp) : Timedesc.t =
-  let local_tz = Timedesc.Time_zone.local_exn () in
   match ts with
-  | `Local t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:local_tz
+  | `Local t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:!target_tz
   (* 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
+  | `Utc t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:!target_tz
   | `With_tzid (ts, (_b, tz_name)) ->
-      (* Qui il timestamp è SCRITTO come se fosse UTC (+00:00) ma in realtà va interpretato con
+      (* The timestamp is stored as if it were UTC but must be interpreted in tz_name.
-       il fuso orario indicato da tz_name. *)
+         We reconstruct the wall-clock time in tz_name, then convert to target_tz. *)
       let tz = Timedesc.Time_zone.make_exn tz_name in
       let wrong_ts = Timedesc.Utils.timestamp_of_ptime ts in
       let date = Timedesc.date (Timedesc.of_timestamp_exn ~tz_of_date_time:Timedesc.Time_zone.utc wrong_ts) in
       let year, month, day = (Timedesc.Date.year date, Timedesc.Date.month date, Timedesc.Date.day date) in
       let time = Timedesc.time_view (Timedesc.of_timestamp_exn ~tz_of_date_time:Timedesc.Time_zone.utc wrong_ts) in
       let hour, minute, second = (time.Timedesc.Time.hour, time.Timedesc.Time.minute, time.Timedesc.Time.second) in
-      Timedesc.make_exn ~year ~month ~day ~hour ~minute ~second ~tz ()
+      let t_in_named_tz = Timedesc.make_exn ~year ~month ~day ~hour ~minute ~second ~tz () in
+      (* Convert from tz_name to target_tz *)
+      Timedesc.of_timestamp_exn ~tz_of_date_time:!target_tz (Timedesc.to_timestamp_single t_in_named_tz)
 
 let timedesc_of_utc_or_timestamp_local (ts : utc_or_timestamp_local) : Timedesc.t =
-  let local_tz = Timedesc.Time_zone.local_exn () in
   match ts with
-  | `Local t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:local_tz
+  | `Local t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:!target_tz
   (* 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
+  | `Utc t -> t |> Timedesc.Utils.timestamp_of_ptime |> Timedesc.of_timestamp_exn ~tz_of_date_time:!target_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