feat(monthly): add support for MONTHLY recurrence (P07, P08)

- Add `monthly_pattern`, `simple_monthly` types and `monthly` field to
  `rem`
- Implement `render_monthly` and `add_until_monthly` in `remind.ml`
- Handle `BYMONTHDAY` (P07) and nth-weekday `BYDAY` (P08) patterns in
  `eventPredicates.ml`
- Add `add_months` utility for date arithmetic
- Mark P07 and P08 as implemented in documentation

bin/eventPredicates.ml

@@ -58,7 +58,7 @@ open Utils
     snippet: 'REM FROM 2025-10-01 UNTIL 2025-10-10 AT 08:30 MSG Daily'
     priorita: Subito
 
-  - id: P07
+  - id: P07 ✅
     pattern: Ricorrenza mensile per giorno fisso
     ics: "RRULE:FREQ=MONTHLY;BYMONTHDAY=…"
     remind_support: nativo
@@ -66,7 +66,7 @@ open Utils
     snippet: 'REM 15 AT 10:00 FROM 2025-01-01 MSG Fatture'
     priorita: Dopo
 
-  - id: P08
+  - id: P08 ✅
     pattern: Ricorrenza “n-esimo weekday” del mese
     ics: "RRULE:FREQ=MONTHLY;BYDAY=MO;BYSETPOS=3"
     remind_support: espansione
@@ -323,6 +323,38 @@ RRULE: (`Weekly, (Some `Until (`Utc (2026-07-01 09:00:00 +00:00))), None, [])
                 Remind.weekly = Some { count_or_until; interval; byday = days; week_start };
               }
       end
+  | Some (_, (`Monthly, count_or_until, interval, recurs)) ->
+      begin match interval with
+      | Some n when n > 1 ->
+          Printf.eprintf "Warning: MONTHLY INTERVAL=%d not supported\t\t\tUID: %s\n" n (Utils.get_uid ev);
+          debug_print_of_recurrence_and_skip ev (`Monthly, count_or_until, interval, recurs)
+      | _ -> (
+          let bymonthday =
+            List.find_map
+              (function
+                | `Bymonthday (d :: _) -> Some d
+                | _ -> None)
+              recurs
+          in
+          let byday =
+            List.find_map
+              (function
+                | `Byday pairs -> List.find_map (fun (n, wd) -> if n <> 0 then Some (n, wd) else None) pairs
+                | _ -> None)
+              recurs
+          in
+          let pattern =
+            match (bymonthday, byday) with
+            | _, Some (n, wd) -> Some (Remind.By_nth_weekday (n, wd)) (* BYDAY takes precedence *)
+            | Some day, None -> Some (Remind.By_month_day day)
+            | None, None -> Some (Remind.By_month_day (Timedesc.Date.day rem.Remind.date))
+          in
+          match pattern with
+          | None ->
+              Printf.eprintf "Warning: MONTHLY with unsupported BYDAY\t\t\tUID: %s\n" (Utils.get_uid ev);
+              debug_print_of_recurrence_and_skip ev (`Monthly, count_or_until, interval, recurs)
+          | Some p -> Ok { rem with Remind.monthly = Some { count_or_until; interval; pattern = p } })
+      end
   | Some (_, recurs) -> debug_print_of_recurrence_and_skip ev recurs
   | None -> Ok rem
 

bin/remind.ml

@@ -17,6 +17,17 @@ type simple_daily = {
 }
 (** A simple daily REM command *)
 
+type monthly_pattern =
+  | By_month_day of int  (** P07: BYMONTHDAY=n or implicit day from DTSTART *)
+  | By_nth_weekday of int * Icalendar.weekday  (** P08: BYDAY=nWD, n≠0, can be negative *)
+
+type simple_monthly = {
+  count_or_until : Icalendar.count_or_until option;
+  interval : int option;
+  pattern : monthly_pattern;
+}
+(** A simple monthly REM command *)
+
 type rem = {
   source : string;  (** Source file or identifier for the reminder *)
   original_uuid : string;  (** Original UID from the iCalendar event *)
@@ -26,6 +37,7 @@ type rem = {
   time : Timedesc.Time.t option;  (** Optional time specification (hour, minute) *)
   duration : Timedesc.Span.t option;  (** Optional duration for timed events *)
   yearly : (int * int) option;  (** Optional simple yearly recurrence (month, day) *)
+  monthly : simple_monthly option;  (** Optional simple monthly recurrence *)
   weekly : simple_weekly option;  (** Optional simple weekly recurrence *)
   daily : simple_daily option;  (** Optional simple daily recurrence *)
   recurring : Icalendar.event list;
@@ -46,6 +58,7 @@ let empty =
     time = None;
     duration = None;
     yearly = None;
+    monthly = None;
     weekly = None;
     daily = None;
     recurring = [];
@@ -111,6 +124,22 @@ let add_until_daily b rem (d : simple_daily) =
       let until = Timedesc.Date.add ~days:((count - 1) * iv) rem.date in
       Buffer.add_string b (spf "UNTIL %s " (Timedesc.Date.to_rfc3339 until))
 
+let add_until_monthly b rem (m : simple_monthly) =
+  match m.count_or_until with
+  | None -> ()
+  | Some (`Until dt) ->
+      let ts = timedesc_of_utc_or_timestamp_local dt in
+      let date = until_date_adjusted ts rem.time in
+      Buffer.add_string b (spf "UNTIL %s " (Timedesc.Date.to_rfc3339 date))
+  | Some (`Count count) ->
+      let base = Utils.add_months rem.date (count - 1) in
+      let until =
+        match m.pattern with
+        | By_month_day _ -> base
+        | By_nth_weekday _ -> Timedesc.Date.add ~days:6 base (* weekday can shift up to 6 days *)
+      in
+      Buffer.add_string b (spf "UNTIL %s " (Timedesc.Date.to_rfc3339 until))
+
 let add_at b = function
   | Some t -> Buffer.add_string b (spf " AT %s" (string_of_time t))
   | None -> ()
@@ -199,6 +228,31 @@ let render_weekly rem (w : simple_weekly) =
   close_omit_context b rem.exdate;
   Buffer.contents b
 
+let render_monthly rem (m : simple_monthly) =
+  let b = Buffer.create 256 in
+  add_omit_context b rem.exdate;
+  add_rem b;
+  add_info b rem.original_uuid;
+  add_source b rem.source;
+  (match m.pattern with
+  | By_month_day day -> Buffer.add_string b (spf "%d " day)
+  | By_nth_weekday (n, wd) when n > 0 ->
+      let day = ((n - 1) * 7) + 1 in
+      add_weekday b wd;
+      Buffer.add_string b (spf "%d " day)
+  | By_nth_weekday (n, wd) (* n < 0 *) ->
+      let back = -n * 7 in
+      add_weekday b wd;
+      Buffer.add_string b (spf "1 --%d " back));
+  Buffer.add_string b (spf "FROM %s " (Timedesc.Date.to_rfc3339 rem.date));
+  add_until_monthly b rem m;
+  add_skip b rem.exdate;
+  add_at b rem.time;
+  add_duration b rem.duration;
+  add_msg b rem.summary;
+  close_omit_context b rem.exdate;
+  Buffer.contents b
+
 let render_single rem =
   let b = Buffer.create 256 in
   add_rem b;
@@ -229,10 +283,13 @@ let string_of_rem rem =
     | None -> (
         match rem.weekly with
         | Some w -> render_weekly rem w
+        | None -> (
+            match rem.monthly with
+            | Some m -> render_monthly rem m
             | None -> (
                 match rem.yearly with
                 | Some (month, day) -> render_yearly rem month day
-            | None -> render_single rem))
+                | None -> render_single rem)))
   in
   let overrides = List.map render_single rem.overrides in
   String.concat "" (main :: overrides)

bin/utils.ml

@@ -160,6 +160,20 @@ let get_rdates ev =
     Printf.eprintf "Found RDATE with periods: %d entries; UID: %s\n" (List.length periods) uid;
   []
 
+let add_months (date : Timedesc.Date.t) (n : int) : Timedesc.Date.t =
+  let year = Timedesc.Date.year date in
+  let month = Timedesc.Date.month date in
+  let day = Timedesc.Date.day date in
+  let total_months = (year * 12) + (month - 1) + n in
+  let new_year = total_months / 12 in
+  let new_month = (total_months mod 12) + 1 in
+  let rec try_day d =
+    match Timedesc.Date.Ymd.make ~year:new_year ~month:new_month ~day:d with
+    | Ok date -> date
+    | Error _ -> try_day (d - 1)
+  in
+  try_day day
+
 let get_recurrence_id ev =
   List.find_map
     (fun prop ->