Qlik Sense Point In Time YTD

[chrisjackson]

Morning all

Can anyone suggest how I can adapt the set analysis below to include the value of a variable "vPoint_In_Time" which will allow a user to enter a date into the variable (using the extension qsVariable) which will then change the YTD based on that variable date?

SUM({$<Date = {'>=$(=YearStart(Today()-1,0,4))<=$(=Date(Today()-1))'}>}Sales)

I'm guessing you have to change the "Today()" part but I can't seem to find the right context to replace it with the variable.