Events synced via CalDAV from Zimbra have end time before start time

[AlexisSantin]

Issue Summary

When an event is synchronized via CalDAV in Zimbra, the event’s end time is set before the start time, resulting in a "negative duration".
In the example observed, the start time is 10:00 AM while the end time is 9:00 AM on the same day, even though the chosen start time in cal.com is 9.00 am and the time slot is 15 minutes. It then displays the event as starting at 10.00 am and ending at 9.00 am, so lasting “-1 hour”, which is logically impossible.

This issue has been observed by multiple users within the same organization using zimbra and cal.com, not just a single account.

Steps to Reproduce

1. Create or synchronize a calendar event in Zimbra.
2. Ensure Zimbra is connected to Cal.com via CalDAV.
3. Open the synchronized event in Zimbra.
4. Observe the event’s start and end times in the Cal.com interface.

Additional context

• The intended duration is 15 minutes.
• The issue appears after synchronization, not during manual event creation in Cal.com.

Actual Results

• The end time is earlier than the start time.
• The UI allows or displays an invalid time range without validation or correction.

Expected Results

• The end time should always be after the start time.

• A 15-minute slot should be displayed as a 15-minute duration.

• Cal.com should either:

  • Correct invalid time ranges coming from CalDAV, or
  • Block and clearly flag invalid synchronized events.

Technical Details

• Calendar provider: Zimbra
• Synchronization method: CalDAV
• Affected platform: Zimbra
• Browser: Firefox (latest version)
• OS: Windows 10

Evidence

• Screenshot attached showing:

  • Start: 1/5/2026 – 10:00 AM
  • End: 1/5/2026 – 9:00 AM
  • Resulting negative duration

This issue was reproduced by several users in the same organization, all using Zimbra synchronized via CalDAV.

Important notice

On the details view we have the problem defined earlier.
But on the calendar view, the same event is shown correctly as you can see :

[linear]

CAL-7198

[Shailesh2302]

Hi 👋

Thanks for the detailed report. After reviewing the CalDAV integration code, it appears the issue may be related to timezone handling during event parsing. In certain cases (notably with Zimbra), start and end times may be re-parsed after ical.js has already applied timezone conversion, which could potentially shift the end time before the start time and result in a negative duration.

I’d like to investigate this further and work on a fix. Could a maintainer please assign this issue to me?

Thanks!