Hey there engrProf,
Thanks for reaching out to the Apple Support Communities. Just to confirm, which account are the contacts that are showing the incorrect birthday being synced from?
Also to help with isolating and troubleshooting the issue further, we'd recommend testing the issue in safe mode, and with a new admin user account. Testing the issue in safe mode can help to determine if the issue is caused by some software that loads as your MacBook starts up. Testing the issue in a new user account can help to determine if the issue is system wide, or only occurring in your user account.
For steps on how to boot into safe mode, check out the steps below from the following resource: How to use safe mode on your Mac - Apple Support
How to use safe mode
Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:
Apple silicon
1. Shut down your Mac.
2. Turn on your Mac and continue to press and hold the power button until you see the startup options window.
3. Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
4. Log in to your Mac. You might be asked to log in again.
Intel processor
1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac.
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.
After testing the issue in safe mode, be sure to go through the additional steps to help troubleshoot whether the issue does or doesn't occur.
For steps on how to create a new user account for testing, check out this next link here: Set up users, guests, and groups on Mac - Apple Support
After testing the issue in the new account, you can go ahead and delete the account. Keep us updated with the results.
Take care!
