I agree with Jeff that a date should be stored as a date in the source system (OLTP). I would try to change it. However, I know how hard it is sometimes to get buy in at the Business Level.

I have work on a few data warehouses (OLAP) in my time. One part of a star schema is a date dimension. Usually some number is given to a date. That number is used throughout the tables.

How about a date decode table that translates the 201301 (a number stored as a char(6)) to '2013-01-01' as a date. Thus, adding one more join to the equation allows you to work with dates.

I would check Aaron's article on handling dates.

http://sqlblog.com/blogs/aaron_bertrand/archive/2009/10/16/bad-habits-to-kick-mishandling-date-range-queries.aspx

I would keep away from BETWEEN.

Good luck ...