how can i select months between two year dates

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SQL-Server

SQL-Server-2008

i want to Use Moth_Diff in my c# application but it does not exist in a table it only exist in this procedure.It returns a null value in tha Application interface

USE [DUT_Governance_Department]
GO
/****** Object:  StoredProcedure [dbo].[StudentsRespective_Studlog]    Script Date: 09/26/2010 10:30:17 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER Proc [dbo].[StudentsRespective_Studlog]
(@User_names char(8)

 )
as
Declare @tela int = CONVERT(int,GETDATE(),111)

select Students.User_names,F_name,Vote_status,Reg_status,Reg_year ,Regions.Region_name,Function_ID.Function_id,DATEDIFF(MONTH, Reg_year ,@tela)as Month_Diff
from Students,Regions,User_Account,Faculty,Campus,Function_ID   
where  Students.User_names =User_Account .User_names 
AND Faculty.faculty_id =Students.faculty_id
AND Campus.Campus_id=Faculty.Campus_id
AND Regions.Region_id=Campus.Region_id
AND Function_ID .Function_id =User_Account.Function_id 
AND Students.Vote_status in('N','Y')
AND Function_ID.Function_id =4
AND Students.User_names =@User_names