I have run into an issue I never noticed previously: I just changed from a "crashed" Windows Vista system to a new Windows 7 Home Premium system. If the problem occurred previously, I didn't notice it, but I do see it now and, conceivably, it might be related to Daylight Savings Time (DST) (which ended early this morning here in the US). Of course, Windows doesn't consistently and logically deal with DST and by far the majority of my photos are taken while DST is in effect, but I often work on them after DST is no longer in effect!
So, two major changes occurred after my previous uses of XnView during which I did not notice this issue (but which I cannot guarantee never happened!

New PC with new operating system,
and
Change from DST to non-DST.
The issue occurs In the Change Timestamp dialog box,
with "Use" selected to be "EXIF: Date taken" (my normal setting I have used for a couple of years or longer)
and
with "Change" selected to be "File date modified",
the shown result is exactly one hour later than that shown for "EXIF: Date taken".
However, the actual result reported by Windows after clicking "Apply" does not appear to have this error!

Thanks!
JERRY