From ced81835ef76f98089348ed9f85e044ba976a562 Mon Sep 17 00:00:00 2001
From: Stefan Hildebrandt <695494+hildebrandttk@users.noreply.github.com>
Date: Sun, 16 Aug 2026 21:52:28 +0200
Subject: [PATCH] fix(calendar): drop default SKIP=OMIT from recurrence rules
 (DAVx5 sync)

Recurring events defaulted skip:"omit"+rscale:"gregorian" on every rule,
which Stalwart serialises into the CalDAV RRULE as RSCALE=GREGORIAN;SKIP=OMIT.
DAVx5 rejects that ("Invalid recurrence rule"), breaking Android sync. Those
RFC 7529 fields only matter for non-Gregorian scales / invalid-date handling,
so stop emitting them for plain Gregorian rules.

- lib/recurrence-rule.ts: createRecurrenceRule() helper with no skip/rscale.
- CalendarRecurrenceRule.rscale/skip are now optional.
- event-modal + recurrence-editor build rules via the helper.
- demo fixture cleaned; unit test asserts no skip/rscale.

Closes #805.
---
 components/calendar/event-modal.tsx       | 33 +++++--------------
 components/calendar/recurrence-editor.tsx | 19 ++---------
 lib/__tests__/recurrence-rule.test.ts     | 40 +++++++++++++++++++++++
 lib/demo/fixtures/calendars.ts            |  2 --
 lib/jmap/types.ts                         |  7 ++--
 lib/recurrence-rule.ts                    | 33 +++++++++++++++++++
 6 files changed, 89 insertions(+), 45 deletions(-)
 create mode 100644 lib/__tests__/recurrence-rule.test.ts
 create mode 100644 lib/recurrence-rule.ts

diff --git a/components/calendar/event-modal.tsx b/components/calendar/event-modal.tsx
index 943da7143..d4eaa5099 100644
--- a/components/calendar/event-modal.tsx
+++ b/components/calendar/event-modal.tsx
@@ -11,6 +11,7 @@ import { RecurrenceEditor, buildRecurrenceSummary, isSimpleRecurrenceRule } from
 import { parseDuration, getEventColor } from "./event-card";
 import { buildAllDayDuration, getEventDisplayEndDate, getEventEndDate, getEventStartDate, getPrimaryCalendarId } from "@/lib/calendar-utils";
 import { displayNow, getEffectiveTimeZone } from "@/lib/timezone";
+import { createRecurrenceRule } from "@/lib/recurrence-rule";
 import { ParticipantInput, type ParticipantInputHandle } from "./participant-input";
 import {
   isOrganizer,
@@ -543,25 +544,7 @@ export function EventModal({
     if (recurrence === "custom" && customRule) {
       data.recurrenceRules = [customRule];
     } else if (recurrence !== "none" && recurrence !== "custom") {
-      data.recurrenceRules = [{
-        "@type": "RecurrenceRule",
-        frequency: recurrence,
-        interval: 1,
-        rscale: "gregorian",
-        skip: "omit",
-        firstDayOfWeek: "mo",
-        byDay: null,
-        byMonthDay: null,
-        byMonth: null,
-        byYearDay: null,
-        byWeekNo: null,
-        byHour: null,
-        byMinute: null,
-        bySecond: null,
-        bySetPosition: null,
-        count: null,
-        until: null,
-      }];
+      data.recurrenceRules = [createRecurrenceRule(recurrence)];
     } else if (event && event.recurrenceRules?.length) {
       data.recurrenceRules = null;
       if (event.recurrenceOverrides) data.recurrenceOverrides = null;
@@ -1383,9 +1366,9 @@ export function EventModal({
                 >
                   {t("events.delete")}
                 </Button>
-                <Button 
-                  variant="ghost" 
-                  size="sm" 
+                <Button
+                  variant="ghost"
+                  size="sm"
                   onClick={() => setShowDeleteConfirm(false)}
                   className="w-full"
                 >
@@ -1417,10 +1400,10 @@ export function EventModal({
             </Button>
           )}
         </div>
-        
+
         {!showDeleteConfirm && (
         <div className="flex gap-2 w-full">
-          <Button 
+          <Button
             variant="outline"
             onClick={isEdit ? () => setMode("view") : onClose}
             className="w-full"
@@ -1428,7 +1411,7 @@ export function EventModal({
             {t("form.cancel")}
           </Button>
           <Button
-            onClick={handleSave} 
+            onClick={handleSave}
             disabled={!title.trim() || isSaving}
             className="w-full"
           >
diff --git a/components/calendar/recurrence-editor.tsx b/components/calendar/recurrence-editor.tsx
index 6bec12f5f..a14669112 100644
--- a/components/calendar/recurrence-editor.tsx
+++ b/components/calendar/recurrence-editor.tsx
@@ -6,6 +6,7 @@ import { addYears, format } from "date-fns";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import type { CalendarRecurrenceRule } from "@/lib/jmap/types";
+import { createRecurrenceRule } from "@/lib/recurrence-rule";
 
 type EditorFrequency = "daily" | "weekly" | "monthly" | "yearly";
 type MonthlyMode = "day" | "nth";
@@ -210,25 +211,11 @@ export function RecurrenceEditor({ rule, eventStart, onSave, onCancel }: Recurre
   };
 
   const buildRule = (): CalendarRecurrenceRule => {
-    const built: CalendarRecurrenceRule = {
-      "@type": "RecurrenceRule",
-      frequency,
+    const built: CalendarRecurrenceRule = createRecurrenceRule(frequency, {
       interval: Math.max(1, interval),
-      rscale: "gregorian",
-      skip: "omit",
-      firstDayOfWeek: "mo",
-      byDay: null,
-      byMonthDay: null,
-      byMonth: null,
-      byYearDay: null,
-      byWeekNo: null,
-      byHour: null,
-      byMinute: null,
-      bySecond: null,
-      bySetPosition: null,
       count: endsMode === "after" ? Math.max(1, count) : null,
       until: endsMode === "on" && untilDate ? `${untilDate}T23:59:59` : null,
-    };
+    });
 
     if (frequency === "weekly") {
       const days = weekDays.length ? weekDays : [startDay];
diff --git a/lib/__tests__/recurrence-rule.test.ts b/lib/__tests__/recurrence-rule.test.ts
new file mode 100644
index 000000000..d1da93e4f
--- /dev/null
+++ b/lib/__tests__/recurrence-rule.test.ts
@@ -0,0 +1,40 @@
+import { describe, it, expect } from 'vitest';
+import { createRecurrenceRule } from '@/lib/recurrence-rule';
+
+describe('createRecurrenceRule (#805 — no default SKIP=OMIT)', () => {
+  it('does not emit rscale or skip (which serialise to RSCALE=GREGORIAN;SKIP=OMIT)', () => {
+    const rule = createRecurrenceRule('yearly');
+    expect('skip' in rule).toBe(false);
+    expect('rscale' in rule).toBe(false);
+    expect(rule.skip).toBeUndefined();
+    expect(rule.rscale).toBeUndefined();
+  });
+
+  it('builds a clean Gregorian default rule', () => {
+    expect(createRecurrenceRule('daily')).toEqual({
+      '@type': 'RecurrenceRule',
+      frequency: 'daily',
+      interval: 1,
+      firstDayOfWeek: 'mo',
+      byDay: null,
+      byMonthDay: null,
+      byMonth: null,
+      byYearDay: null,
+      byWeekNo: null,
+      byHour: null,
+      byMinute: null,
+      bySecond: null,
+      bySetPosition: null,
+      count: null,
+      until: null,
+    });
+  });
+
+  it('applies overrides (interval / count / until) without reintroducing skip', () => {
+    const rule = createRecurrenceRule('yearly', { interval: 2, count: 100 });
+    expect(rule.interval).toBe(2);
+    expect(rule.count).toBe(100);
+    expect(rule.until).toBeNull();
+    expect('skip' in rule).toBe(false);
+  });
+});
diff --git a/lib/demo/fixtures/calendars.ts b/lib/demo/fixtures/calendars.ts
index 86affaf7c..68f493512 100644
--- a/lib/demo/fixtures/calendars.ts
+++ b/lib/demo/fixtures/calendars.ts
@@ -161,8 +161,6 @@ export function createDemoCalendarEvents(): CalendarEvent[] {
         '@type': 'RecurrenceRule',
         frequency: 'weekly',
         interval: 1,
-        rscale: 'gregorian',
-        skip: 'omit',
         firstDayOfWeek: 'mo',
         byDay: [{ day: 'mo' }],
         byMonthDay: null,
diff --git a/lib/jmap/types.ts b/lib/jmap/types.ts
index 6198afb27..ce0ae02c1 100644
--- a/lib/jmap/types.ts
+++ b/lib/jmap/types.ts
@@ -673,8 +673,11 @@ export interface CalendarRecurrenceRule {
   '@type': 'RecurrenceRule';
   frequency: 'yearly' | 'monthly' | 'weekly' | 'daily' | 'hourly' | 'minutely' | 'secondly';
   interval: number;
-  rscale: string;
-  skip: 'omit' | 'backward' | 'forward';
+  // Optional and normally omitted: these RFC 7529 (RSCALE/SKIP) fields only
+  // apply to non-Gregorian scales / invalid-date handling. Emitting a default
+  // SKIP=OMIT breaks some CalDAV clients (DAVx5) - see lib/recurrence-rule.ts (#805).
+  rscale?: string;
+  skip?: 'omit' | 'backward' | 'forward';
   firstDayOfWeek: 'mo' | 'tu' | 'we' | 'th' | 'fr' | 'sa' | 'su';
   byDay: CalendarNDay[] | null;
   byMonthDay: number[] | null;
diff --git a/lib/recurrence-rule.ts b/lib/recurrence-rule.ts
new file mode 100644
index 000000000..607edf156
--- /dev/null
+++ b/lib/recurrence-rule.ts
@@ -0,0 +1,33 @@
+import type { CalendarRecurrenceRule } from '@/lib/jmap/types';
+
+type Frequency = CalendarRecurrenceRule['frequency'];
+
+/**
+ * A plain Gregorian recurrence rule with all by-parts unset. `rscale`/`skip`
+ * are intentionally omitted: they only matter for non-Gregorian scales /
+ * invalid-date handling, and defaulting them made Stalwart serialise
+ * `RSCALE=GREGORIAN;SKIP=OMIT` into the RRULE, which DAVx5 rejects (#805).
+ */
+export function createRecurrenceRule(
+  frequency: Frequency,
+  overrides: Partial<CalendarRecurrenceRule> = {},
+): CalendarRecurrenceRule {
+  return {
+    '@type': 'RecurrenceRule',
+    frequency,
+    interval: 1,
+    firstDayOfWeek: 'mo',
+    byDay: null,
+    byMonthDay: null,
+    byMonth: null,
+    byYearDay: null,
+    byWeekNo: null,
+    byHour: null,
+    byMinute: null,
+    bySecond: null,
+    bySetPosition: null,
+    count: null,
+    until: null,
+    ...overrides,
+  };
+}