Hey Paul,
Thank you for reaching out to us!
I’m so sorry to hear that you are having problems with your calendar: let me help you to solve this!
Can you please make sure that both Events Calendar PRO (v.4.4.18) and The Events Calendar (v.4.6.1) are up-to-date?
I’ve ran some tests in my own WordPress install with all Modern Tribe plugins enabled and couldn’t replicate the issue from here: if I edit one of my past events and update the date everything works as expected.
When did you notice this issue started to happen? Was it after a recent plugin update?
If you access your Dashboard and head over to Settings -> Permalinks for a permalink flush (no need to change anything, just visit the permalinks page: WordPress will automatically flush your permalinks once you visit it), does it help you to solve the problem?
I hope this helps! Let me know if you have any other questions and I’ll be happy to assist!
Best Regards,
Patricia