Setting date criteria in form lookup field

Setting date criteria in form lookup field

I am new to Zoho Creator and am having trouble with one of my forms.

The form includes a lookup field for students to register for a particular class (thos data is drawn from the Scheduled Classes view). Currently the form shows all classes. However, I only want students to be able to see classes scheduled for the future.

I tried adding the criteria Start_Date >= zoho.currentdate to this field in my form, but it's not working.

Can anyone tell me what I am doing wrong?

Thanks,
Amy