Filename with a defined DateRange in it

Hi Gurus, 

I'm trying to change the file names of  txt files using SSIS expression. But I'm not sure how I can go back 4-2 months from current date. Ex: Today's Date is 8/19/2013. So, If my job runs today or this month,

then my fiile name should be like:

 File1_20130401_20130630.txt

If my job runs next month it should be : File2_20130501_20130731.txt

If you observe the dates it's  like 3 months data and I want start & end dates for that period in the filename.

Please help me with this.Thanks