In iOS 12, contacts stored in your Exchange account might show birthdays that are one day off. To fix this issue, update to iOS 12.2 or later, then redownload the contacts from your Exchange account:
- Update to iOS 12.2 or later.
- Go to Settings > Passwords & Accounts, then tap your Exchange account.
- Turn Contacts off, then tap Delete. This removes the Exchange contacts on your device that you'll download again in the next step.
- Turn Contacts on.
Contacts from your Exchange account automatically redownload to your device with the correct birthday.