Like daylight saving time being or not being applied? Have you looked at the timezone settings in Thunderbird and more importantly in your operating system? Just because the computer shows the correct time does not mean it is set for the correct locale so that things like daylight saving are correctly applied.

Thanks for the response! I just double-checked, and the time zone is correct (New York) across macOS, Thunderbird, and the events. As well as being the timezone for iOS and the web calendar. The events on the Exchange calendar are also in the New York timezone.

I might be wrong, but It doesn't seem to be a daylight savings time issue. The event times for repeating events are consistent across the DST changeover.

Can you see time zone of your event ?

Yes, the time zone of my event is America/New York as expected.

I can across similar issue myself. I created an event which would occur in the future but importantly - after the daylight correction was either enabled or disabled. It meant all those events would appear one hour incorrect until daylight saving had occurred at which point the event times were auto corrected. This only effects the calendar not incoming emails.

For timezone America/New York currently it's UTC -4, but alter Sun, 2 Nov, 2025 it will alter to UTC -5 This is that one hour difference. So a 10AM event created on or after 2 Nov 2025, will currently look like 9AM, but will alter to 10AM once the DLS has altered.

Are the events you are creating on or after 2 Nov but before Sun, 8 Mar, 2026?

Might be on to something here. The events I have been creating are before November 2, and some of them are repeating events which cross November 2. These all have the hour-change issue (even the repeats after Nov 2). But when I create a new event from November 2 - March 8, it doesn't seem to have the hour change issue. Events after March 8 do have the hour change issue again.

So right now events only appear correctly if they are created sometime November 2 - March 8. Otherwise they appear incorrectly.

This is probably a different issue, but I tried switching my time zone within Thunderbird to GMT-4, and all the events (still in New York time) ended up showing up as 9 hours (8 hours for the Outlook calendar events) behind what they should be.

Before switching to GMT-4, was Thunderbird configured to "Set timezone manually" to New York time or to "Use System Time" ?

If it was configured to "Set timezone manually" then you may try to switch to "Use System Time" to see.

It seems to make no difference whether I use the system timezone or manually set the time zone.