Two Outlooks with two CalDavSynchronizers on a shared calender do play ping-pong (SEQUENCE is increasing)

[winnieXY]

Hey,

I'm using a nextcloud with a shared calender for me and my wife. We both sync the shared calender into our outlook using CalDavSynchronizer.

To prevent Timezone Pingpong i switched on the option "Create events on server with downloaded IANA timezones" and selected our timezone.

However this does not prevent the ping-pong (and thus notifications about changed appointments from nextcloud).
This happens for all appointments within the calender.

Here is one example of such a appointment - once synced by me, once synced by my wife:

After my CalDavSynchronizer synced:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//ddaysoftware.com//NONSGML DDay.iCal 1.0//EN
BEGIN:VTIMEZONE
LAST-MODIFIED:20251211T094730Z
TZID:Europe/Berlin
TZURL:https://www.tzurl.org/zoneinfo-outlook/Europe/Berlin
X-LIC-LOCATION:Europe/Berlin
BEGIN:DAYLIGHT
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=3
TZNAME:CEST
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
END:DAYLIGHT
BEGIN:STANDARD
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
TZNAME:CET
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
CLASS:PUBLIC
DTEND;VALUE=DATE:20251215
DTSTAMP:20260115T082251Z
DTSTART;VALUE=DATE:20251212
PRIORITY:5
SEQUENCE:743
SUMMARY:Test Appointment
TRANSP:TRANSPARENT
UID:3e99ec25-1c42-47a2-876d-d1d0b4804d0b
X-MICROSOFT-CDO-BUSYSTATUS:FREE
END:VEVENT
END:VCALENDAR

after the caldavsynchronizer of my wife synced:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//ddaysoftware.com//NONSGML DDay.iCal 1.0//EN
BEGIN:VTIMEZONE
LAST-MODIFIED:20251211T094730Z
TZID:Europe/Berlin
TZURL:https://www.tzurl.org/zoneinfo-outlook/Europe/Berlin
X-LIC-LOCATION:Europe/Berlin
BEGIN:DAYLIGHT
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=3
TZNAME:CEST
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
END:DAYLIGHT
BEGIN:STANDARD
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
TZNAME:CET
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
CLASS:PUBLIC
DTEND;VALUE=DATE:20251215
DTSTAMP:20260115T080954Z
DTSTART;VALUE=DATE:20251212
PRIORITY:5
SEQUENCE:742
SUMMARY:Test Appointment
TRANSP:TRANSPARENT
UID:3e99ec25-1c42-47a2-876d-d1d0b4804d0b
X-MICROSOFT-CDO-BUSYSTATUS:FREE
END:VEVENT
END:VCALENDAR

The ping-pong is also very well visible within the apache access log:

195.145.170.173 - - [14/Jan/2026:13:11:50 +0000] "PUT /remote.php/dav/calendars/user1/gemeinsamer-kalender/3e99ec25-1c42-47a2-876d-d1d0b4804d0b.ics HTTP/1.1" 204 - "-" "CalDavSynchronizer/4.6"
87.191.45.66 - - [14/Jan/2026:13:43:53 +0000] "PUT /remote.php/dav/calendars/user2/gemeinsamer-kalender_shared_by_user1/3e99ec25-1c42-47a2-876d-d1d0b4804d0b.ics HTTP/1.1" 204 - "-" "CalDavSynchronizer/4.6"
195.145.170.173 - - [14/Jan/2026:14:43:09 +0000] "PUT /remote.php/dav/calendars/user1/gemeinsamer-kalender/3e99ec25-1c42-47a2-876d-d1d0b4804d0b.ics HTTP/1.1" 204 - "-" "CalDavSynchronizer/4.6"
94.31.74.44 - - [14/Jan/2026:16:25:49 +0000] "PUT /remote.php/dav/calendars/user2/gemeinsamer-kalender_shared_by_user1/3e99ec25-1c42-47a2-876d-d1d0b4804d0b.ics HTTP/1.1" 204 - "-" "CalDavSynchronizer/4.6"
147.161.235.1 - - [15/Jan/2026:07:07:43 +0000] "PUT /remote.php/dav/calendars/user1/gemeinsamer-kalender/3e99ec25-1c42-47a2-876d-d1d0b4804d0b.ics HTTP/1.1" 204 - "-" "CalDavSynchronizer/4.6"
87.191.45.66 - - [15/Jan/2026:08:08:40 +0000] "PUT /remote.php/dav/calendars/user2/gemeinsamer-kalender_shared_by_user1/3e99ec25-1c42-47a2-876d-d1d0b4804d0b.ics HTTP/1.1" 204 - "-" "CalDavSynchronizer/4.6"
147.161.235.1 - - [15/Jan/2026:08:22:51 +0000] "PUT /remote.php/dav/calendars/user1/gemeinsamer-kalender/3e99ec25-1c42-47a2-876d-d1d0b4804d0b.ics HTTP/1.1" 204 - "-" "CalDavSynchronizer/4.6"

The only difference is the DTSTAMP and a increased SEQUENCE (which will trigger the notification mail from nextcloud).

would it be possible to add a option to caldavsynchronizer to stop updating the appointment if just the DTSTAMP has changed (or updated the DTSTAMP without increasing SEQUENCE?

We both have this setup:

• Windows: Win 11
• Outlook 365
• CalDavSynchronzier 4.6.0
• Nextcloud 31

If needed I could get some debug logs.