Can I change incorrect 'picture taken on' date/time?

On a recent trip, I forgot to reset the camera date/time to reflect a 7
hour time zone change. Therefore, all of the pictures taken have an
incorrect date/time stamp.
Because I traveled from East to West, some of the pictures taken in the
evening, are listed as being taken the following day, in the early
morning. Disconcerting!

Is there any way to correct this date group which is stored with each
picture?

Additional info
I have software to modify the date/time of file properties "Created on",
"Modified on", and "Accessed on", but this does not change the picture's
internal data.
Picture format is .jpg, camera is HP R707, which stores camera settings
with each photo. Computer uses Windows XP.
Camera type shouldn't matter, as almost all digital cameras store the
date/time with the image.
Forgetting to adjust the time, would probably be a common error, made by
many people, and a utility to correct the picture's internal date/time
would be useful.

Can anyone help?