CorrectICS

Why Calendar Imports Break After Migration

If you switched calendar providers and the import went badly, you are not dealing with a rare edge case.

Calendar migrations break all the time for very predictable reasons:

  • duplicate events
  • broken recurring events
  • wrong times or timezone drift
  • oversized ICS files
  • old metadata from previous migrations

The frustrating part is that the import step looks simple on the surface. Export from one system, import into another, done.

In reality, many calendar files carry years of history and compatibility baggage.

If your migration is already broken, start with /fix.

1. Duplicate events after migration

Duplicates usually come from one of three patterns:

  • importing the same file more than once
  • mixing a subscription with a one-time import
  • older event identifiers or calendar history confusing the destination system

Once duplicates appear, repeated retries often make the problem worse.

Related: ICS Import Created Duplicate Events – How to Clean It Up.

2. Recurring events behave differently across platforms

Recurring events are one of the hardest parts of calendar migration.

Even if the source calendar looked fine, the destination system may interpret parts of the recurring history differently, especially when the file includes:

  • long-running recurring series
  • exceptions and overrides
  • old imported data
  • reminder-heavy events

This is one reason historical work calendars often migrate less cleanly than expected.

3. Timezone shifts and wrong-hour events

A migration can also surface timezone issues that were easy to miss in the original system.

Common causes:

  • invalid TZID values
  • missing VTIMEZONE blocks
  • floating times mixed with explicit timezone data
  • historical daylight saving time complexity

Related: Fix ICS Timezone Errors (Events at the Wrong Time).

4. Oversized or historically messy ICS files

Some migrations fail because the file is simply not a small, tidy export.

Warning signs:

  • the .ics file is much larger than expected
  • the calendar contains many years or decades of history
  • the file has clearly moved across providers before
  • imports fail in vague or inconsistent ways

In those cases, you are often looking at a file that needs repair before it can migrate cleanly.

A real example: How We Repaired a 30-Year Google Workspace Calendar for Migration to Fastmail.

5. Manual sanity checks you can do first

Before you assume the whole migration is lost, do a few quick checks:

  • confirm you exported the right source calendar
  • keep the original .ics as a backup
  • import only once into a test calendar if possible
  • check recurring events, all-day events, and older historical entries
  • stop retrying if you see duplicates, wrong times, or partial imports

These checks will not fix every problem, but they help you avoid making the situation worse.

6. When the file needs real repair help

You probably need repair help when:

  • multiple symptoms appear at once
  • the file is too large for realistic manual inspection
  • the destination calendar accepts the file but results are clearly wrong
  • you cannot trust the event history after import

That is where CorrectICS fits. It gives you a validation step and a repair path before repeated imports multiply the damage.

Start here: /fix.

7. Destination-specific migration help

If you are moving away from Google Workspace, use the guide that matches your destination:

8. Next step

If the calendar import broke after migration, do not keep guessing and do not keep importing the same file blindly.

Fix your .ics file in seconds

Upload an iCalendar file and get a clean, import-ready version for Google Calendar, Outlook, Apple Calendar, and Teams.

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