1/20/2011 in your expression is interpreted as 1 divided by 20 divided by 2011 as / is the division operator. You need to change the expression so that it is interpreted as a date instead.
Get ( CurrentDate) > GetAsDate ( "1/20/2011" )
or Get ( CurrentDate ) > Date ( 1 ; 20 ; 2011 )
Thank you very much!