Incorrect timezone definition in ICS invitations causes 1-hour shift for recipients

Calendar
1

Hi eM Client-Team,

I found what appears to be a timezone-related issue in calendar invitations generated by eM Client on macOS.

Environment

  • eM Client 10.4.5326 (97a2e75)
  • macOS
  • CalDAV calendar (Nextcloud)
  • Local timezone: Europe/Berlin

Problem

When I create and send a meeting invitation from eM Client, recipients receive the appointment one hour later than intended.

Example:

  • Event created for 25 June 2026, 11:30–12:00
  • Organizer sees the correct time
  • Event is stored correctly in the CalDAV backend
  • Recipients see the event shifted by +1 hour

Outlook reports that the invitation was sent using timezone Europe/Oslo.

Generated ICS

The invitation generated by eM Client contains:

BEGIN:VTIMEZONE
TZID:Europe/Oslo

BEGIN:STANDARD
DTSTART:20261025T030000
...
END:STANDARD

BEGIN:DAYLIGHT
DTSTART:20270328T020000
...
END:DAYLIGHT
END:VTIMEZONE

DTSTART;TZID=Europe/Oslo:20260625T113000

The event is in June 2026, but the DAYLIGHT definition starts in March 2027.

Could this cause some clients to interpret the event as UTC+01 instead of UTC+02 during summer time?

Has anyone seen similar behaviour or can confirm whether this VTIMEZONE definition is correct?

Thanks!