In an effort to keep this effort going I have grabbed a screen shot of two renderings of the same index.
This is a vanilla vB 3.8.2 with only the openX integration applied.
The window on the left is a template with the <ad.../> tags invoked. The window on the left shows the same index with a clean style.

Notice that the time is correct in both cases as stated by the board time at the bottom of the main page - this is the native time zone for the board since even though I am logged in my user timezone is the same as the server. Guests would see exactly the same in each case.
The Posts and Most Users Online dates (and PMs etc) all show back at UTC with the view_ad function called - but not the date at the bottom and importantly only for display - not effecting in any way the accuracy of the clock for database purposes (thankfully)
I think therefore that I need to find out how to protect whatever value/variable is being used to calculate the times that are being written out
after the view_ad function call.
The way I tried to do this is to save a value, then reset it before exiting the plugin but either it didn't work or it was the wrong variable etc etc etc.
I know I am blogging this but I would still appreciate any insight or assistance from a more skillful hand if one is available to look at the problem.
The board in question can be accessed and is my dev board.
Will