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I've had a some correspondence with the author. He says that IStats uses the server timezone to enter data into the database via functions in the counter.php file.
I think, if there's a way to offset the time with the default XOOPS timezone as it's entered into the database, the problem would be solved.
I had a look at the XOOPS API docs and there is a UserTimeToServerTime() function available (but not a ServerTimeToUserTime() function! maybe it could be used to convert the time before input to the database.
In the meantime, I'm using Istats, but stats based on my timezone would be more meaningful to me. I await future developments.