SQL Server 2012 :: Day Wise And Date Range Calculation With Looping Or Dynamic Data?

I am using Sql Server 2012.

This is how I calculate the ratio of failures in an order:

31 Days Table 1 query
sum(CASE
WHEN (datediff(dd,serDATE,'2015-01-21')) >= 31 THEN 31
WHEN (datediff(dd,serDATE,'2015-01-21')) < 0 THEN 0
ELSE (datediff(dd,serDATE,'2015-01-21'))END) as 31days1 .

How do i loop and pass dates dynamically in the Datediff?

31 Failures Table 2 query
SUM(Case when sometable.FAILUREDATE BETWEEN dateadd(DAY,-31,CONVERT(DATETIME, '2015-01-21 23:59:00.0', 102))
AND CONVERT(DATETIME, '2015-01-21 23:59:00.0', 102)Then 1 Else 0 END) As Failures31,31 Day Cal(Formula) combining both Table 1 and Table 2
((365*(Convert(decimal (8,1),T2.Failures31)/T1.31day))) [31dayCal]This works fine when done for a specific order.

I want a similar kind of calculation done for day wise and month wise.

2. what approach should I be using to achieve day wise and month wise calculation?

I do also have a table called Calender with the list of dates that i can use.