Prefix - set effective date using SQL Expresssion

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Hello - I'm hoping someone can help me with this please.

I have an OTBI analysis that I want to schedule on the first of each month that will show changes in the previous month. One of the 'changes' could be that the record has been ended but because the because the subject area is a Real Time one (Payroll - Element Entries Real Time) the record is no longer current when the report is being run so it will not report the change. 

Therefore, I am wondering if I could include the SQL expression below (to get to the 1st of the previous month) in the prefix to set the effective date to be the 'first of the previous month'.