Hi Varun,
Add_Months function will add specified number of months to a Date, as you want to go back 1 year (12 months), we're doing a MINUS of 12 months. Make sense?
Also, if the previous year was leap year and if the date is 2/28/2013, then it will go back one year to 2/28/2012 (which is a leap year).
Thanks,
Mahboob Mohammed