MSSQL - DATEDIFF not accurate

Im trying to retrieve the month difference of two dates but it seems like i cant find a way to get the accurate months.

Here are the queries i tried so far :

SELECT DATEDIFF(month,convert(datetime, '11/05/2015'), convert(datetime, '12/06/2015')) - 1

This will result to 0 which is wrong and when i used another date :

 SELECT DATEDIFF(month,convert(datetime, '12/31/2015'), convert(datetime, '01/01/2016')) - 1

This would yield to 1.

Leap year must also be considered.