something I am using has a concept of "locale" as you put it , whether it is Xoops, Icalendar or Outlook.

if this was not the case then the time would not change by an hour, which is the difference between UK GMT & BST (British Summer Time).

AS for settings, in Xoops the server time is set to -GMT-5:00 and the default time is GMT-00:00
this is because the server is in the states and we are in the UK, there was an issues with Xoops 2.3 with the British time zone but this was corrected in 2.4
and now the time display correctly.

In calendar "General Settings" there is an option to set the "summer/Winter"
1: Value of Xoops config
2: Value of winter time as told by server (Recommended)
3: Value of summer time as told by server

this was set to No 2 "Value of winter time as told by the server" I now plan to set this as "Value of Xoops config" and see what happens.

I will post results later.