How to incorporate dates with set analysis?

[#1]

I need to include a date parameter and an additional calculation within my existing set anlaysis expression. I had originally try to include it in a varilable but that was causing problems. My current expression using the variable is below:

if(aggr(count({$ <ObserveDate = {">=$(varDateParameter)"}, ChoiceCTCustomerDescription = >} ChoiceCTReferenceId) - count({$ <ObserveDate = {">=$(varDateParameter)"}>} ChoiceCTReferenceId) <= 0, StaffRecnum), StaffName, null())

varDateParameter code is below:

num(varLookBackDate +(only (FacilityTimezoneOffset) / 24) - (varHours / 24))

I need to figure out how to get the second part of code into the set analysis expression without using a variable. Any help would be greatly appreciated. Thank you!