Big T

Hi,
I am trying to find an expression builder formula that will do the following
in a form:

When Install Date is less or equal to today, I want the check box to be
checked automatically.
This is what I have so far but it's not working: =IIf([Install Date]=>
[Date],"",[Installed]=Yes).

Now I have just started working with expression builder so I could be
missing some obvious components. Help!