Calculating a Rolling Date

Try

=IF(MONTH(A16)<>MONTH(WORKDAY(A16,1)),WORKDAY(DATE(YEAR(A16)-1,MONTH(A16)+1,0),-1),WORKDAY(DATE(YEAR(A16)-1,MONTH(A16),DAY(A16)),-1))

Try this formula, where your original date is in A1

=WORKDAY(IF(MONTH(A1)<>MONTH(WORKDAY(A1,1)),EOMONTH(A1,-12),EDATE(A1,-12))+1,-1)

Note: this uses Analysis ToolPak functions WORKDAY, EOMONTH and EDATE so you need the toolpak installed. If formula gives an error try installing with

Tools> add-ins > tick "Analysis ToolPak"