Mailbox migration error “Invalid property [Calc:StartTime] StartTime – invalid or missing DateTime value” (4381582)

During mailbox migration, the task fails on a calendar item with the following error:

At least one property failed validation.
Invalid property = [Calc:StartTime] StartTime.
PropertyErrorDescription: Property [Calc:StartTime] StartTime is required and must contain a valid DateTime value.

The user’s mailbox contains a corrupted calendar item where the hidden property Calc:StartTime is missing or contains invalid data. This typically occurs with old meeting requests (legacy Outlook, or data from old migrations).

1. Identify the calendar item using the subject and date from the error details.
2. Delete the problematic calendar item from the user’s calendar.
3. Re-run the migration.

Deleting the corrupted item resolves the issue in most cases.