Help: trouble with criteria in form lookup field

I am new to Zoho.

I'm customizing a form for students to register for specific classes. In the form I have a lookup field for students to select the class they want to attend (this field pulls data from a "scheduled classes" view).

The form lookup field current shows all scheduled classes. I want to limit the drop-down to show only classes with a "Start_Date" in the future.

I have tried setting the following criteria in this field: Start_Date >= zoho.currentdate

But, this is not working. Can anyone tell me what I am doing wrong?

Thanks,
Amy