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There are generally 2 kinds of calendars you may already have - your own personal one, and invites or shares of a few "central" calendars the Co-op has maintained.
The central ones will be managed centrally and will be reshared to your new MS365 account once you are fully onboarded.
For personal ones, it is HIGHLY recommended that you move your work calendaring to Outlook, as it will become the central calendar solution for scheudling across the Co-op.
1. If you visit https://support.microsoft.com/en-us/office/import-google-calendar-to-outlook-098ed60c-936b-41fb-83d6-7e3786437330 the first step is an accurate description of how to get a calendar *export* out of Google Calendar. However the second step to import it is dated.
2. To do that, go to Outlook online (https://outlook.office.com/calendar/). On the left-hand side choose "Add Calendar"
and in the ensuing window, select "Upload from File" and slect the google calendar archive you have extracted in the previous step.
While it is possible to "subscribe" to an external calendars feed in Outlook, this is different than importing it and is not recommended.
===== Importing your Thunderbird Calendar Into Outlook
If you follow these instructions to export your calendar in iCalendar format:
https://support.mozilla.org/en-US/kb/exporting-and-sharing-a-calendar
You should be able to import it into your calendar on M365:
Importing your calendar from Horde to Outlook
These instructions will help you get your calendar .ics file exported out of Horder
https://support.cpanel.net/hc/en-us/articles/1500007805721-Exporting-a-calendar-in-Horde
You should then be able to import it into your calendar on M365: