To help you build such date formulas I have assembled a set of useful calculations for you to use.
Note: To use the calculations in Filter Modifiers you simply replace "getdate()" with the desired filter placeholder, e.g. "{Value1}".
First Day of Month
DATEADD(mm, DATEDIFF(mm,0,getdate()), 0)
Monday of the Current Week
DATEADD(wk, DATEDIFF(wk,0,getdate()), 0)
First Day of the Year
DATEADD(yy, DATEDIFF(yy,0,getdate()), 0)
First Day of the Quarter
DATEADD(qq, DATEDIFF(qq,0,getdate()), 0)
Midnight for the Current Day
DATEADD(dd, DATEDIFF(dd,0,getdate()), 0)
Last Day of Prior Month
DATEADD(dd,-1,DATEADD(mm, DATEDIFF(mm,0,getdate() ), 0))
Last Day of Prior Year
DATEADD(dd,-1,DATEADD(yy, DATEDIFF(yy,0,getdate() ), 0))
Last Day of Current Month
DATEADD(dd,-1,DATEADD(mm, DATEDIFF(m,0,getdate() )+1, 0))
Last Day of Current Year
DATEADD(dd,-1,DATEADD(yy, DATEDIFF(yy,0,getdate() )+1, 0))
First Monday of the Month
DATEADD(wk, DATEDIFF(wk,0, dateadd(dd,6-datepart(day,getdate()),getdate())), 0)
First Day of the Month 12 Months Ago
DATEADD(mm, DATEDIFF(mm,0,getdate()) - 12, 0)
Note: You can change "12" to any number of months ago you wish.