select
	ST = convert(varchar(10),ST,121),
	DT = convert(varchar(10),DT,121),
	DAYS_THIS_MONTH =
	case
	-- Compare to last day of month
	when DT > dateadd(mm,datediff(mm,-1,getdate()),-1)
	-- Difference between start date and last day of month
	then datediff(dd,ST,dateadd(mm,datediff(mm,-1,getdate()),-1))
	-- Difference between start date and last date
	else datediff(dd,ST,DT)
	end
from
	(select ST = convert(datetime,'2006-06-10')  )b
	cross join
	(
	Select DT = convert(datetime,'2006-06-27') union all
	Select DT = convert(datetime,'2006-07-27') 
	) a

ST         DT         DAYS_THIS_MONTH 
---------- ---------- --------------- 
2006-06-10 2006-06-27 17
2006-06-10 2006-07-27 20

(2 row(s) affected)