Get days of a month in SQL

@dt is a datetime value
 
derive the first of this month, add a month to it
then subtract 1 day to get the last day of this month
 

select days_of_month=day(
        dateadd(d,-1,
            dateadd(m,1,
                dateadd(d,1-day(@dt), @dt)
                )
            )
        )