CorrectICS

Fix ICS Timezone Errors (Events at the Wrong Time)

If you imported an .ics file and all your events are one hour off, shifted to a different day, or show in the wrong timezone, you’re almost certainly looking at a timezone (TZID / VTIMEZONE) problem inside the file.

This guide explains:

  • What “wrong time” ICS issues look like in Google Calendar, Outlook, and Apple Calendar
  • The most common timezone mistakes inside .ics files
  • When you can safely fix the file yourself vs. when to use an autofix tool

If you just want the file repaired right now: Try CorrectICS Autofix.

1. Typical symptoms of ICS timezone errors

You might see one or more of these:

  • Events show one hour earlier or later than expected
  • Events move to the previous or next day after import
  • All‑day events suddenly become multi‑day events
  • Recurring events appear in the wrong local time after DST changes
  • One participant sees the correct time, another sees something different

These are almost never “calendar bugs” — they’re usually missing or invalid timezone data in the .ics file.

2. The most common ICS timezone mistakes

Behind the scenes, ICS files use a combination of DTSTART, DTEND, TZID, and VTIMEZONE blocks. When these are wrong, calendars guess — and that’s where the time shifts happen.

Common problems:

  • TZID=EST or TZID=PST instead of a valid name like America/New_York
  • Using TZID=GMT-5 or other non‑standard identifiers
  • Missing VTIMEZONE block even though TZID is used
  • A DTSTART;TZID=... that doesn’t match any timezone defined in the file
  • Mixing floating times (no timezone) with explicit timezones

You can see these by opening the .ics file in a text editor and searching for DTSTART, DTEND, and VTIMEZONE.

For a deep dive into the exact field formats, see the more technical guide:
ICS Time Zone Errors (TZID, VTIMEZONE, UTC vs Local).

3. Quick, non‑technical fix: upload and autofix

If you’re not comfortable editing raw ICS text, the fastest route is:

  1. Go to /fix on CorrectICS.
  2. Upload your .ics file.
  3. Review the timezone‑related warnings (look for TZID / VTIMEZONE).
  4. If available, click “Download fixed file” and re‑import it.

CorrectICS looks for:

  • Invalid or unrecognized TZID values
  • Missing VTIMEZONE definitions
  • Obvious timezone mismatches that can be normalized safely

It will either:

  • Normalize the timezone definitions where safe, or
  • Surface clear errors so you (or your developer) can fix them manually

4. Manual fix (for developers)

If you maintain the system that generates the .ics files:

  1. Switch to Olson/IANATZ names
    Use identifiers like America/New_York, Europe/Berlin, Asia/Tokyo, not EST, PST, or CET.

  2. Include a matching VTIMEZONE block for each TZID you use
    For example, if you have:

    DTSTART;TZID=America/New_York:20250310T090000

    make sure there is a VTIMEZONE component with TZID:America/New_York defined in the file.

  3. Be consistent across DTSTART, DTEND, and RECURRENCE‑ID
    Don’t mix DTSTART:20250310T090000Z with DTEND;TZID=America/New_York:... unless you understand the implications.

  4. Test with real clients
    After making changes, import the file into:

    • Google Calendar (web)
    • Outlook (desktop or web)
    • Apple Calendar (on macOS)

    Confirm that the event time is correct in each.

For more detailed engineering guidance, see:
Generate Correct ICS Files (Best Practices + Templates).

5. When to use an autofix tool vs. hand‑editing

Use an autofix tool like CorrectICS when:

  • You received an .ics from someone else and just need it to work
  • You don’t control the system that generated the file
  • You’re debugging a one‑off issue for a customer

Consider manual fixes or code changes when:

  • You own the application that exports the .ics files
  • Multiple customers report “wrong time” issues
  • You want a permanent fix instead of a case‑by‑case repair

In both cases, running the file through the validator at /fix is a good first step — you’ll see exactly which lines and fields are causing trouble.

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