Try using DATEDIFF

http://msdn.microsoft.com/en-nz/library/ms189794%28v=sql.100%29.aspx

In conjunction with ROUND

http://msdn.microsoft.com/en-us/library/ms175003%28v=sql.100%29.aspx

For example:

declare @work_order_end_date date

declare @work_order_start_date date

set @work_order_start_date = '2012-12-03'

set @work_order_end_date = '2013-05-31'

select round(datediff(d, @work_order_start_date, @work_order_end_date) / 30.0, 0)