Incorrect syntax near the keyword 'convert' in stored procedure using sql server 2005

Hi,

I have the following code that is talking to a sql stored procedure: -

USE [testdata]
GO

DECLARE      @return_value int,
            @NewTestID uniqueidentifier,
            @ErrorCode int,
            @ErrorDescription varchar(255)

EXEC      @return_value = [dbo].[AddNewTestData_v0141]
            @Operator = 13,
            @SNumber = 1689,
            @SerialNumber = N'A08171470380006',
            @DateTimeStart = convert(datetime,'2008-04-22 13:35:00',120),
            @DateTimeStop = convert(datetime,'2008-04-22 13:36:00',120),
            @TestResult = N'1',
            @Assembly = N'AH387775U001',
            @StageRefNum = 945,
            @TransactionNo = 2,
            @NewTestID = @NewTestID OUTPUT,
            @ErrorCode = @ErrorCode OUTPUT,
            @ErrorDescription = @ErrorDescription OUTPUT

SELECT      @NewTestID as N'@NewTestID',
            @ErrorCode as N'@ErrorCode',
            @ErrorDescription as N'@ErrorDescription'

SELECT      'Return Value' = @return_value

GO


It keeps on coming up: -

Msg 156, Level 15, State 1, Line 11
Incorrect syntax near the keyword 'convert'.

The two date parameters are set to datetime.

What am I doing wrong please?

Many Thanks
Lee