I was able to fix the same error by adjusting the site collection time zone to match the time zone of the group calendar.
Insert Group Calendar WebPart causing error
A user reported this error when trying to add a group Calendar in a SharePoint Online Modern Page
1- From any page on the site, Edit.
2- Hit Plus sign in between section to add a web part.
3- Select "Group Calendar"
Something went wrong
If the problem persists, contact the site administrator and give them the information in Technical Details.
When you click on TECHNICAL DETAILS:
Something went wrong
If the problem persists, contact the site administrator and give them the information in Technical Details.TECHNICAL DETAILS
Cannot read property 'offset' of undefined
TypeError: Cannot read property 'offset' of undefined
at t._setRegionalSetting (https://spoprod-a.akamaihd.net/files/sp-client/sp-groupcalendar-webpart_en-us_801607585d6000d87d72.js:1:72238)
at t.onInit (https://spoprod-a.akamaihd.net/files/sp-client/sp-groupcalendar-webpart_en-us_801607585d6000d87d72.js:1:63868)
at t._onInViewport (https://modern.akamai.odsp.cdn.office.net/files/sp-client/sp-pages-assembly_en-gb_0cae5c87d44603e42b4c1842a2fec810.js:65:1622211)
at new Promise (<anonymous>)
at t._internalFirstTimeRender (https://modern.akamai.odsp.cdn.office.net/files/sp-client/sp-pages-assembly_en-gb_0cae5c87d44603e42b4c1842a2fec810.js:65:1620082)
at e._renderWebPart (https://modern.akamai.odsp.cdn.office.net/files/sp-client/sp-pages-assembly_en-gb_0cae5c87d44603e42b4c1842a2fec810.js:65:1653637)
What I tried
- I created a calendar on the site as it had none but this made no difference. (so not an issue related to not having calendar)
- Trying adding the group calendar WebPart on a new page within the site causes the same problem. (so not page related)
- Tried adding webpart on another site in the same tenancy and did not get the error. (so must be site related)
- I have checked site features and compared to a site where it does work found no differences.
I have been looking at this for too long and thought I would put it out here in case I am missing something obvious.