Your syntax is a little off. First, you can't use >= or <= where you are. There, you need a Set Operator. Replace those with equal signs.
You're also missing the brackets around your Set Modifiers.
Then you need to do the greater than or equal and the date expression inside a dollar sign expansion. Dates seem to be the most difficult to get right when doing Set Analysis for me. I usually end up trying out a few different things before getting it right.