One folder contains 14 images which are all stamped in the EXIF:Date taken field 21 hours, 55 minutes, 14 seconds ahead of actual.
First attempt: I selected one image, chose EXIF:Date Taken at the top of the dialog box; clicked on the 3rd radio button, selected EXIF:Date Taken, chose -1 day, and set the time to 2:04:46 (on the theory that the date and the clock time seemed to be resetting independently of each other).
I arrived at that conclusion (there is nothing in the HELP file for timestamp) because only the date box has a +/- selection. The clock set box has no + or - and it only sets on a 12 hour basis, not a 24 hour basis.
Needless to say, that attempt didn't work. It set the date back 2 days and the time back (not forward) 2 hours, 4 min, 46 sec.
Second attempt: tried setting the date increment to minus zero and the time increment to 21 hours, 55, minutes, 14 sec (the actual differential that I was seeking). That didn't work either.
Obviously, there is a detail here that I am missing.

I'm using version 1.91.3 on a fully updated WinXP system.
Assistance would be appreciated -- I would like to be able to use XnView more! Thanks.