How can I display a past date on my project?

Hi folks, here's what I'm trying to find a solution to, please:

I want the system's current date minus 21 years.  So, for example, today's date is 09/10/2013 (MM/DD/YYYY) and I would like the option to have the date of 21 years ago displayed as 09/10/1991.  Is there a way I can create this function in the variables or advanced options?

Thank you,

Justin