Buggy TZID values used in Exchange shared calendar file (ICS)

Question

Buggy TZID values used in Exchange shared calendar file (ICS)

Sulev Reisberg 0

User can share their Exchange calendar as ICS file. However, the time zones in the file (TZID) are not put together correctly by Microsoft.

In the iCalendar (RFC 5545) specification, TZID can be any string identifier, but it only works reliably if:

It’s a well-known IANA time zone name (e.g., Europe/Paris).
Or it matches exactly one of the VTIMEZONE blocks defined in the same file.

However, the ICS file produced by Outlook does not follow this rule which causes problems.

For example, my calendar has the following event:

DTSTART;TZID=Romance Standard Time:20250822T140000

DTEND;TZID=Romance Standard Time:20250822T150000

But:

"Romance Standard Time" is not in the IANA tz database and are not guaranteed to be understood by other calendar systems
and it is NOT descibed in the header of ICS file as VTIMEZONE which looks like following:

BEGIN:VCALENDAR

METHOD:PUBLISH

PRODID:Microsoft Exchange Server 2010

VERSION:2.0

X-WR-CALNAME:Kalender

BEGIN:VTIMEZONE

TZID:W. Europe Standard Time

BEGIN:STANDARD

DTSTART:16010101T030000

TZOFFSETFROM:+0200

TZOFFSETTO:+0100

RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10

END:STANDARD

BEGIN:DAYLIGHT

DTSTART:16010101T020000

TZOFFSETFROM:+0100

TZOFFSETTO:+0200

RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3

END:DAYLIGHT

END:VTIMEZONE

BEGIN:VTIMEZONE

TZID:FLE Standard Time

BEGIN:STANDARD

DTSTART:16010101T040000

TZOFFSETFROM:+0300

TZOFFSETTO:+0200

RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10

END:STANDARD

BEGIN:DAYLIGHT

DTSTART:16010101T030000

TZOFFSETFROM:+0200

TZOFFSETTO:+0300

RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3

END:DAYLIGHT

END:VTIMEZONE

BEGIN:VEVENT

Because of this incorrect use, the other calendars fail to interpret the event's time correctly.

Correct usage would be:

Either change Outlook export to use IANA zones (TZID=Europe/Paris etc.), or
Make sure every Windows TZ string that appears (like Romance Standard Time) has a matching VTIMEZONE definition.

Microsoft has to fix this.

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Answer 1

Hi Sulev Reisberg,

Thank you for posting your question in the Microsoft Q&A forum.

Based on your description, I understand you're reporting a bug related to the timezone format in ICS files exported from Outlook. Your breakdown of the issue is very clear and easy to follow.

I’ve looked into this issue then did some further research and found that you're not alone — other users have encountered similar problems, and it can indeed be frustrating, especially when events are misinterpreted by non-Microsoft calendar services.

I truly appreciate the effort you've made to document this so thoroughly. Your insights are valuable and will certainly help others facing the same challenge here in the forum.

To help move this forward, I’d recommend submitting this issue directly to the Microsoft Feedback Portal, where it can be more easily seen by the product team. In fact, I found a feedback post that closely matches your issue:

Shared .ics file does not include 'Romance Standard Time' zone in VTIMEZONE definitions, causing ev…

I highly encourage you to vote on it and add your insights in the comments. The more users who report and engage with the same issue, the higher the chance it will be acknowledged and addressed by Microsoft.

Once again, thank you for taking the time to analyze and share this — your contribution is truly appreciated!

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

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Buggy TZID values used in Exchange shared calendar file (ICS)

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