Exporting and importing a Google Calendar
After a calendar is migrated from Microsoft Outlook it will probably show up as a new calendar and not update the primary Google Calendar with the migrated events. Google Calendar offers a way to export and import calendars using .ICS (preferred) or .CSV files as a way to resolve this problem. You could also use these instructions to export or import calendar events to other services or devices.
To export a Google Calendar:
On your computer, open Google Calendar. You can't export events from the Google Calendar app on a smartphone or tablet.
On the left side of the page, find the "My calendars" section.
Click the Down arrow to the right of the calendar you want to export and select "Calendar settings" from the menu.
Click "Export this calendar" option toward the bottom of the window.
A ZIP file that contains an ICS file of your events will start downloading.
Once the download is complete open the downloaded ZIP file and copy the ICS file inside it to another location such as the Windows Desktop so the file is ready for import.
After you've exported your events, you can import them into your primary Google Calendar.
Open Google Calendar on a computer. Note: You can only import from a computer, not a phone or tablet.
In the top right, click the Gear icon and select "Settings".
Open the "Calendars" tab.
Click "Import calendar" which is between the "My calendars" and "Other Calendars" sections.
Click "Choose File" and select the file you exported. The file should end in .ICS or .CSV.
Choose which calendar to add the imported events to. By default, events will be imported into your primary calendar.
Adapted from "Import events to Google Calendar"