Adding A Date In A Text File?

I have a program i wrote today, it works just as i wanted it to, with exception of one problem. Im more looking for some ideas on this one. For my work, we have folders on a drive that are filled with txt files, what we wanted to do was to add the last date it was modified to the top of the file so people would know when it was last modified. These files are used on CNC machines and the those people cant look at a last modified date as easy as the computer user can, just a short explanation of why we wanted to do this. So i wrote a program to do it, loop through each file in the user selected folder and add a date to the top of it, works perfect. Now here is the issue, when i write the last mod date to that file, the last mod date on the file changes to that day. For example, if i have a file in there that was modified January 28th 2011, the program reads that file, then on the top of the file it prints that date, January 28th 2011, but then that same file gets changed to the current date as a modified date, so if the program i wrote is used again on the same folder the next day, those files will all end up getting the previous days date on the top. I dont want that! I guess im wondering if there is a way to tell the files mod date not to get updated when its run through my program, it should only update when changed some other way, like someone actually opens it, changes stuff and hits the save button.