Current selection impacts set analysis

[bala_sundar]

Hi,

I have created the below set function to avoid current selection on date but it still impacts. 

Created set to show last 30 day average but when i select a date in filter, the set value gets changed according to selected date.

Please help to fix this.

avg({$<Date={">$(=Date(Today()-31,'MM/DD/YYYY '))"}>} sales)