Today including time info : getdate()
Today without time info : DATEADD(DAY, DATEDIFF(day, 0, getdate()), 0)
Tomorrow without time info : DATEADD(DAY, DATEDIFF(day, 0, getdate()), 1)
Beginning of current month : DATEADD(month, datediff(month, 0, getdate()), 0)
Beginning of last month : DATEADD(month, datediff(month, 0, getdate())-1, 0)
así que lo más probable
WHERE dateColumn >= DATEADD(month, datediff(month, 0, getdate())-1, 0)
AND dateColumn < DATEADD(DAY, DATEDIFF(day, 0, getdate()), 1)