If I add this ICS file to Outlook or add it to my third party calendar app it creates an appointment at 17-dec-2017 at 11:30-12:30, which was the intention.

 

If I upload the ICS file in webmail, it creates the appointment at 10:30-11:30 which is one hour to early.

Is this a DST issue?

 

ICS file testcase:

 

BEGIN:VCALENDAR
VERSION:2.0
X-WR-CALNAME:Test
BEGIN:VTIMEZONE
TZID:NL
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
UID:201712171130@mytest
DTSTART;TZID=NL:20171217T103000Z
DTEND;TZID=NL:20171217T113000Z
SUMMARY:Test appointment at 11:30-12:30
END:VEVENT
END:VCALENDAR