I tried all sorts of things to fix this, including replacing the timezone.bgl file from the installation disks. Then I looked at the default flight and found a section called [DateTimeSeason]" Per that section, I had saved my default flight in the Spring, after DST went into effect. Aha!

[DateTimeSeason]
Day=2
Year=2010
Hours=8
Minutes=10
Seconds=51
Season=Winter
Prior to the new default file, the season was Spring. That fixed the offset for GMT! I vaguely recall doing this procedure in the distant past, but being chronologically chalenged, I had forgotten until after I stumbled on the solution once again.
Nigel²