SQL Server Arithmetic overflow error converting expression to data type int

Question

I'm getting this error:

msg 8115, level 16, state 2, line 18
Arithmetic overflow error converting expression to data type int.

With this SQL query:

DECLARE @year VARCHAR(4);                       
DECLARE @month VARCHAR(2);                      

-- START OF CONFIGURATION SECTION                       
-- THIS IS THE ONLY SECTION THAT SHOULD BE MODIFIED                     
-- SET THE YEAR AND MONTH PARAMETERS                        

SET @year = '2013';                     
SET @month = '3';  -- 1 = January.... 12 = Decemeber.                       

-- END OF CONFIGURATION SECTION                     

DECLARE @startDate DATE                     
DECLARE @endDate DATE                       
SET @startDate = @year + '-' + @month + '-01 00:00:00';                     
SET @endDate = DATEADD(MONTH, 1, @startDate);                       

SELECT                          
    DATEPART(YEAR, dateTimeStamp) AS [Year]                         
    , DATEPART(MONTH, dateTimeStamp) AS [Month]                         
    , COUNT(*) AS NumStreams                        
    , [platform] AS [Platform]                      
    , deliverableName AS [Deliverable Name]                     
    , SUM(billableDuration) AS NumSecondsDelivered                      
FROM                            
    DeliveryTransactions                        
WHERE                           
    dateTimeStamp >= @startDate                     
AND dateTimeStamp < @endDate                        
GROUP BY                            
    DATEPART(YEAR, dateTimeStamp)                       
    , DATEPART(MONTH, dateTimeStamp)                        
    , [platform]                        
    , deliverableName                       
ORDER BY                            
    [platform]                      
    , DATEPART(YEAR, dateTimeStamp)                         
    , DATEPART(MONTH, dateTimeStamp)                        
    , deliverableName  

Can someone please help me solve this?