Post migration , An error message is displayed stating:

 "Unable to load events of <CALENDAR-NAME> Team Calendar (<EXECPTION-ERROR-ID>):java.lang.RuntimeException: The TimeZone value=<UNKNOWN TIMEZONE> neither can be parsed nor mapped to any of known values 

This is because the original calendar's timezone is not recognized in Cloud, such as:

1. CET
2. SystemV/EST5
3. Etc/GMT 12

Confluence Cloud only use the timezone values available from the Atlassian account preferences dropdown and standard region-based timezone identifiers (for example, Europe/Berlin rather than abbreviations like CET). Reference

 
Notes

This issue arises because the initially configured timezone in the calendar does not exist in Cloud.

This problem requires manual intervention to fix. Implementing a mechanism that automatically maps the original timezones to valid timezones in Cloud during the import/migration process would be beneficial.

Workaround:
No bulk operation. Manually edit timezone for the calendar in cloud.