Conversion of 0 or other numbers directly to datetime is documented in SQL Server Books Online.

select

DT,

FirstOfMonth = dateadd(mm,datediff(mm,0,a.DT),0),

LastOfMonth = dateadd(mm,datediff(mm,-1,a.DT),-1)

from

( --Test Data

select DT = getdate() union all

select DT = '20080214 14:37:25.867'

) a

Results:

DT FirstOfMonth LastOfMonth

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

2009-09-03 11:23:58.503 2009-09-01 00:00:00.000 2009-09-30 00:00:00.000

2008-02-14 14:37:25.867 2008-02-01 00:00:00.000 2008-02-29 00:00:00.000