Sorry, for being late on this (was waiting for a friend to explain this to me. You know who you are! )

The reason is fairly simple:

There are no 0st days in a month

SELECT CAST(0 AS datetime)

------------------------------------------------------

1900-01-01 00:00:00.000

(1 row(s) affected)

That's the first (day one, not zero). So DATEPART(d,7) = 8 and DATEPART(d,8) = 9...