I believe the DIR function will return the results based upon the system. So I don’t think you can change that through the script.
That being said, an alternative would be to use the shell_exec and then parse the results. Using the shell_exec you could pass the system dependent directory command (i.e. dir for windows, ls for *nix) and the appropriate “Sort” command (i.e. ls -lrt) to achieve the desired results.
If you want to get more complicated, I believe there is also a function that will allow you to “View” the file properties (including date accessed/created). You could then just put the results of the dir function in an array, chug back through and check the file creation date, and the kick out your results accordingly. I believe that function would be filectime. It might also be helpful to look through the other File manipulation Functions