I deleted the calendar app on my iphone and am unable to bring it back??

You can't delete the following Apps from the iPhone

Photos, Camera, Clock, App Store, Health, Wallet, Safari, Phone, Message & Find My apps.

Check for it under the App Library. Next to the last page of the Home Screen. If it is in the App Library Touch, Hold and Drag the icon to Home Screen. That's it.

I just tested it by deleting my calendar app.

Simply redownload it from the app store.