Hi DGUPTA9,
I just checked and you're right there doesn't seem to be a direct way to get the modified date of a file.
As of now the only quickest option I can think of is to add a few lines of code in a vbs or js file that returns the modified date in a variable. Then run that script from AA and use the returned variable further in AA. Let me know if you want me to assist you on this.
If I find out anything directly in AA then I will let you know or if you come across anything then you can post here.