It looks like the following are the valid operators for the "Date" type:
does not equal
is one of
is not one of
old than days
newer than days
I think "after" will do what you want.
Note: To get this list I just create a smart group in the GUI, select something of the same type and then look at the available operators.
This worked perfect. Not sure how I overlooked that one but thanks for pointing it out!