Calendar shows all day event over 2 days?

Other options you want to check on both devices is:

• On iPhone - Go to Settings > Privacy & Security > Locations Services > System Services, and make sure Setting Time Zone is turned on.
• On the Mac - Go to  > System Settings > Privacy & Security > Location Services > System Services (Details), and make sure Setting Time Zone is turned on.
• Then on the Mac, go back to  > System Settings > General > Date & Time, and make sure Set time zone automatically using your location is turned on. On the bottom of that screen, if you see a warning that new Time Zone definitions will be applied after a restart, you will need to restart the Mac, and then review the options for the Closest city.
• Also on the iPhone at Settings > General > Date & Time, on the bottom of that screen if you see a warning saying that new Time Zone definitions will be applied after a restart, you will need to restart the iPhone too.

This issue is caused by a known bug in Apple's local Finder/USB sync engine, where all-day events are misinterpreted across time zones or system architectures, adding an extra day during direct data transfers. 

Try this

Standardize Time Zone Settings

The two devices must handle time zone calculations identically. 

• On your iPhone: Go to Settings > General > Date & Time and ensure Set Automatically is turned ON.
• On your Mac: Go to System Settings > General > Date & Time and ensure Set time and time zone automatically is toggled ON

It did not help. And it wanted to make my time Toronto when I don’t live anywhere near Toronto. I have this issue with all-day events only so holidays and birthdays. Manitoba just stopped daylight savings so think this may have something to do with this…..