I suspect a corrupt message, in particular, it might be missing the Date: and or other required header lines. It could have been sent to you in this condition.

While viewing the message, press Ctrl+U to view the raw message data, or go to menu item (on Windows) <View | Headers | All > to see the raw headers. Your may not know exactly what to look for, but does anything seem to be missing or garbled, especially the Date: line?

Many thanks Lin. Unfortunately this is complicated by time zones. Msg received in New Zealand from England (13 hours behind NZ at this time of year). The local displayed time for the msg in TB currently sits at 20.57, yet I had replied to that msg 4 hours earlier!

Raw data (Ctrl-U) shows From as Feb 21 13.54. Would that be my local time of receipt? If so then that is the correct time for the msg. I replied to it at 16.14, yet as already mentioned TB displays the time as 20.57.

In the raw data the Received and X-Received time/date are PST. Not sure why that should be and not either GMT or NZDST. Possibly the arrival time on the Gmail server?

Any further ideas to correct this annoyance would be most welcome. Thanks again for your help.

I know only a little about how to interpret email headers, but I do know that the From line does not normally contain a date, so if yours does, that's a red flag in of itself. The important question is, does it contain a Date line?

I believe that when you create a message, the current date and time are stamped in the Date header. Each server it passes through on its way to the destination stamps that current date/time in a Received line, oldest toward the bottom of the header block, newest toward the top.

A quick experiment with Thunderbird seems to indicate that if the originating/creation Date line is missing or malformed, Tbird considers the time received to be the sending time, which, of course, can be wildly wrong. The date shown is not actually found in the header, so I think it must be stored in the index. Furthermore, if I move one of those messages to another folder, it will now show that date it was moved as the sending time!

I'm not sure exactly what this all means, but I have a feeling that it's related to your issue of dates that seem to change unpredictably.

I might try creating a new folder, copying all messages in the errant folder into it, and examine the new folder to see it it appears more stable. If so, delete the old folder. You may need to adjust any filters that reference the old folder.