From c156f6f52bb558f74037f9ab787d63c9d889fbdd Mon Sep 17 00:00:00 2001
From: Sean <sean0407@users.noreply.github.com>
Date: Sun, 18 Oct 2026 10:31:52 +0800
Subject: [PATCH] fix: tolerate empty BYMONTHDAY values in tasks

Signed-off-by: Sean <sean0407@users.noreply.github.com>
---
 src/models/task.js                        | 17 ++++++++++++++++-
 tests/javascript/unit/models/task.spec.js | 17 +++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/src/models/task.js b/src/models/task.js
index 0c3313bcf..433e7b4de 100644
--- a/src/models/task.js
+++ b/src/models/task.js
@@ -34,6 +34,21 @@ import {
 	mapRecurrenceRuleValueToRecurrenceRuleObject,
 } from './recurrenceRule.js'
 
+/**
+ * Removes empty values from BYMONTHDAY lists before parsing. Some CalDAV
+ * clients send values such as `BYMONTHDAY=,24`, which ical.js rejects.
+ * @param {string} vcalendar The vCalendar data
+ * @return {string}
+ */
+function normalizeEmptyByMonthDayValues(vcalendar) {
+	return vcalendar.replace(/(RRULE(?:;[^:\r\n]*)?:)([^\r\n]*)/gi, (line, prefix, rule) => {
+		return prefix + rule.replace(/(^|;)BYMONTHDAY=([^;]*)/i, (part, separator, days) => {
+			const normalizedDays = days.split(',').filter(Boolean).join(',')
+			return normalizedDays ? `${separator}BYMONTHDAY=${normalizedDays}` : ''
+		})
+	})
+}
+
 export default class Task {
 
 	/**
@@ -48,7 +63,7 @@ export default class Task {
 			throw new Error('Invalid vCalendar')
 		}
 
-		const jCal = ICAL.parse(vcalendar)
+		const jCal = ICAL.parse(normalizeEmptyByMonthDayValues(vcalendar))
 		if (jCal[0] !== 'vcalendar') {
 			throw new Error('Only one task is allowed in the vCalendar data')
 		}
diff --git a/tests/javascript/unit/models/task.spec.js b/tests/javascript/unit/models/task.spec.js
index a35babb3c..785bcddb2 100644
--- a/tests/javascript/unit/models/task.spec.js
+++ b/tests/javascript/unit/models/task.spec.js
@@ -43,6 +43,23 @@ describe('task', () => {
 		expect(task.isRecurring).toEqual(true)
 	})
 
+	it('Should parse a BYMONTHDAY list with an empty value', () => {
+		const vcalendar = `BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VTODO
+UID:empty-month-day
+DTSTAMP:20261018T000000Z
+DTSTART:20261001T020000Z
+RRULE:FREQ=MONTHLY;BYMONTHDAY=,24;UNTIL=20261225T020000Z
+SUMMARY:Repro
+END:VTODO
+END:VCALENDAR`
+		const task = new Task(vcalendar, {})
+
+		expect(task.isRecurring).toEqual(true)
+		expect(task.recurrenceRule.byMonthDay).toEqual([24])
+	})
+
 	it('Should set status to "COMPLETED" on completion.', () => {
 		const task = new Task(loadICS('vcalendars/vcalendar-default'), {})
 		task.complete = 100