Add up all amount for a given measure

Hi all,

Is it possible to create a variable that will add up all amount from day 1 to the selected period. Lets say day 1 is February 20 2003 and the user selects a period from prompt as Sept 205. Can I adjust my variable to add up all amount from day 1 to date (Current selected date)? Please see a variable that i created giving me for the selected period.

=Sum(if([Source] = "MI"; [Amount]) ForEach([G/L Account];[Data Source]))

I want to it to roll up all balances from day 1 to the amount for each G/L Account and not just give me current period. Where Amount = Measure

All help appreciated. Thanks