How to write expression with variable as field

[#1]

Hi,

I want to write expression using variable.

for eg.

=num(Sum({<BillingDate = {'<=$(=(max(CalenderDate))) >$(=date(max(CalenderDate) -30))'}>} $(varQty)))

In this example,I am using variable as  $(varQty). But this is not showing any data .

[Anonymous]

please provide what you have under $(varQty)