I recomend you to write a small QBASIC program to get and compute the dates (use the DATE$ function). Then the prog can SHELL the necessary commands, or PRINT a batch file to be executed (with the dates already computed and inserted at the appropriate places).

If you still want to use plain DOS batch files, the following site contain links and lots of tricks (includng how to get the date, manipulate strings and make numerical computations); you will definitely find a solution or idea:
http://mh101.infi.net/~wtnewton/batch/batfaq.html