You can use
SetFile on the command line to do this however it is not included by default in Mac OS X 10.4.x (Tiger). If you have installed the Developer Tools or most of the Combo updates it is installed but in a non standard location.
SetFile was included in
/usr/bin/ for Mac OS X 10.5 and later.
To find the command you can try using
locate to find the location.
Assuming you have the Developer Tools installed:
/Developer/usr/bin/SetFile -d '12/31/2008 12:00:00 PM' nameoffile.txt
You can use another program called
GetFileInfo that is installed with
SetFile to obtain the modification date.
An example script for one file:
# Usage: nameOfThisScript.sh nameOfFile.txt
modifiedDate=`/usr/bin/GetFileInfo -m $1`
/usr/bin/SetFile -d "$modifiedDate" $1