As already noted, read up on the "find" command---there are many options.
Before attempting to write a script, be sure you know how the individual commands work. For example, find has specific options for the 3 different file times: Accessed, Modified, and Changed status.
I'm unclear on what you mean "replacing the date"---do you want to actually modify the properties of the file?
Finally, if you have not already read it, go to http://tldp.org
and get the Bash Guide for Beginners