If what you're intending is to "explode" the range (say 1/1/2014 to 6/1/2014), then the easiest way is probably using an auxiliary Tally table. (Mine is called "Numbers" for some stupid reason.)

Here's the query:

SELECT CustomerNbr

, FlagCode

, StartDate

, EndDate

, DATEDIFF(m,StartDate,DATEADD(d,1,EndDate)) AS MosBetween

, SomeNumber

,DATEADD(m,SomeNumber,StartDate) as TheDate

FROM Flags CROSS JOIN SCRIDB.dbo.t_Numbers

WHERE EndDate<='01-Jan-2015'

AND SCRIDB.dbo.t_Numbers.SomeNumber BETWEEN 1 AND DATEDIFF(m,StartDate,DATEADD(d,1,EndDate));

The date math might be a little off, but you get the idea. You cross join the table with dates (that you're calculating from) and the Tally or Numbers table and filter using a BETWEEN/WHERE clause.