I guess you location (locale) and timezone are not correctly specified in your operating system as Thunderbird gets the date and time from that source. Dates and times in email and calendars are stores as UTC times with offsets. So if your locale is not correct in your OS then the +5 stored may become +6 when daylight saving is applied and when it comes to location. Of the opersting system thinks you are in Brazil and you are really in New Zealand it will also be getting dates based on time wrong because the dateline has been crossed. All from the comfort of your chair with incorrect settings.

Thanks - but the Date & Time settings in 'system preferences' are all correct. The small month calendar in the upper left corner is also correct. It's just the headings on the calendar block itself that are wrong, and the times when I try to put in a new event.....

I have the same problem, but in my case in Windows 10. My Thunderbird is up-to-date. I just came to a different time zone (as I regularly do), and so today I reset the Date & Time settings in the system and in Thunderbird, and TB suddenly began displaying incorrect dates in the calendar block. See screen shot. I hope someone can offer a fix.