Hey there, Dans88!
Thanks for using Apple Support Communities! We understand you're seeing the wrong date on your calendar. We'd like to lend a hand so you have correct information displayed on your iPhone!
To clarify, are you seeing this from a calendar widget? If so, is this an Apple widget or a third party app widget? If using an Apple widget, are you able to delete the widget then add it back to your screen? Does the issue persist? If this continued, Contact Apple Support for help with Apple apps.
If you're using a third party app/widget, you can use the steps in the following support article to help with the app:
If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
Should this persist, contact the app developer:
How to contact an app developer - Apple Support
Best wishes!