We can find out the first and last day with time of the previous month by simple step.
The SQL query is shown below
SELECT DATEADD(month, DATEDIFF(month, -1, getdate()) - 2, 0) as FirstDayWithTime,
DATEADD(ss, -1, DATEADD(month, DATEDIFF(month, 0, getdate()), 0)) as LastDayWithTime