Selective changing of file dates

My girlfriend and I are just back from a trip to Africa. We took about
1,500 photos between us, but now they are loaded on my PC, I noticed
that her camera was set to the wrong date. The day and month (also time)
are OK, but she had set the year as 2010, not 2009. This is important
because I want to be able to view the pictures in the order we took them.

I have been looking for ways of changing the year from 2010 to 2009 on
the files affected, without changing the other elements of the date, but
I can't find anything which would help. I don't want to have to do it
manually because over 800 pictures are involved.

Does anyone have any ideas?