How do I increment a value using the regular Windows command-line?

Hello friends.
I need to be able to increase a date in my text program so that I can keep track of the number of days worked during a project. I need to find out how to increment a value using the Windows command-line.
I know there is a way to do this by using a small script that tells the program to increase the value but I am not sure how to do this. I am hoping someone can give me some suggestions on simple ways to go about doing this.
Thank you so much.
