Thunderbird gets its time from your operating system When is it off by exactly an hour you usually have not set the time zone on your computer correctly.

Thank you, but the time zone is set correctly and is automatic. However, if I "set time automatically", it takes one hour off. I have had to change the system time manually, but it works for all other apps. Maybe this is a Windows problem?

My guess is your locale is not set correctly for daylight savings.

use the windows key + R. In the resultant run dialog type control and press enter. Locate the entry for date and time in the control panel window and double click it.

The top half of the new window is for time. The bottom half is for the locale information that sets among other things your timezone and daylight saving information. It also offers a much better information about daylight saving than the normal windows 10 date and time settings app.

Make sure your time zone is set to a city in the same timezone you are. In my case I live in UTC+9:30 so windows lists both Adelaide and Darwin. Both have the same timezone, but if I use the wrong one my daylight savings information will be off. Make sure the automatically adjust for daylight savings is turned on.

After you are done making adjustments, you might now want to go to the "normal" windows 10 date & time tool. I took you this way because the control panel way show what the daylight saving settings are. The windows 10 app hides those details.

The Control Panel version was indeed different from the standard Windows 10 Settings App. Although DST was set in the 10 app, it was not in the Control Panel (although the time zone was correct). I have now set all to automatic and it works. Thank you very much for this advice. I am just about to post another puzzle about how Gmail works with Thunderbird (or not!). Your magic may work here too.