The other question is why use more variables and code than needed?

DECLARE @d SMALLDATETIME

SELECT DATEADD(mm, DATEDIFF(mm, 0, ISNULL(@d, GETDATE())), 0)

SELECT DATEADD(d, -1, DATEADD(mm, 1 + DATEDIFF(mm, 0, ISNULL(@d, GETDATE())), 0))

As a side note, if you're going to do quite a bit of date manipulation you're better off using a calendar table. There are plenty of examples out there so I won't go into all of that here. What if you need the first and last date of every month in a period? You could use a numbers table and date functions but the calendar table is much more efficient.

Just my two cents worth 😉