Mac Calendar & contacts moving birthday ahead one late

I have this same issue too - occurs on Mac OS Apple Calendar and Fantastical Calendar apps on desktop.

All is good on iOS calendar apps.

It appears to be a sync issue between O365 and Apple Contacts app in my opinion which is well documented with no fix in sight -  

https://discussions.apple.com/thread/8534277

https://discussions.apple.com/thread/252265297

https://discussions.apple.com/thread/252172406

https://forums.macrumors.com/threads/contact-app-issues-birthdays-day-off-deleting-mail-addresses.2268092/

Hi, raycpa.

Thank you for posting in the Apple Support Communities.

If we understand correctly, you’re syncing your Office 365 contacts on your Mac, and the birthdays for those contacts are showing exactly one day off from their actual birthdays in Calendar. It sounds like you’ve done some isolation by determining that it shows the correct date in the Office 365, and only changes when viewed in Contacts or Calendars. Have you tried any troubleshooting?

We recommend the following:

• Check to make sure you’re allowing your Mac to set the time zone automatically and determine your location based in Location Services. 

Set the date and time on your Mac - Apple Support

• Remove and Reactivate your Office 365 account on your Mac:

Activate Office for Mac - Office Support

If you’re still not able to get the birthdays from Office 365 to sync correctly, we urge you to reach out to Microsoft Support.

Warmest Regards.

Hi

I have the same problem. The birthday date looks correct in Outlook client on Mac, Iphone and Ipad.

In contact and calendar on Mac it shows birthday one + 1 day.

Thank you.

BR

Edvard

raycpa,

You're very welcome. We understand that the birthday changes when you view it in Contacts or Calendars on your Mac, and if we understand correctly, it's only occurring with content that you are syncing from your Office 365 account.

If you haven't already, you might test by adding a birthday to an iCloud Contact to see if it also adjusts by a day. If it does, you'll know the issue is with your Mac. If it doesn't, and it only shifts with content you have set to sync from Office 365, we know that the issue is with syncing between your Microsoft account and your macOS software. Microsoft will be the best team to help you troubleshoot content syncing from their service.

We were able to locate this response from a Microsoft Agent with instructions for resetting Outlook syncing preferences.

Calendar sync - birthdays are all off by one day - Microsoft Community

Another response from a Microsoft Agent suggests reaching out to their dedicated Outlook 2016 for Mac support.

Outlook help & learning - Microsoft Support

Best Wishes.