You can check the solutions to see if they are working for you.

Besides iPhone calendar, you also use other calendar service, such as iCloud calendar, Google calendar, and Exchange calendar, etc.

And you must always sync your iPhone calendar with these calendar services.

However, sometimes, you may come across an annoying issue that is iPhone calendar not syncing at all after iOS 1211 update.

No matter iPhone calendar not syncing with iCloud, not syncing with Gmail, or not syncing with Outlook, which will be very depressing.

You can check this by going to Settings Apple ID iCloud Calendars.

In the Calendar app, go to Calendars and drag the screen down for a refresh.

Besides, it even allows you to sync data from one iCloud to another iCloud account directly.

Go to Settings Mail, Contacts, Calendars, select your Google account.

Then go back to Calendar, tap on Default Calendar and select Gmail calendar as default.

You can remove your Google account from Mail, Contacts, and Calendars, and then re-add it.

Disabled Calendars on your iPhone, open iTunes on your computer and connect your iPhone.

Click your device and click Info Click Sync Calendars with and select Outlook.

By the way, when facing iPhone calendar not syncing with iCloud, dont forget to have a try on AnyTrans.

Icalendar And Mac For Outlook Not Syncing