SQL Server Error?

I have a stored procedure that I'm trying to compile and I keep getting this error message:

"Implicit conversion from data type datetime to smallint is not allowed. Use the CONVERT function to run this query."

Here is the SP:

ALTER PROCEDURE [dbo].[spAddUpdate_HEADER_Equipment]
	@returnValue			INT=NULL,
	@equipment_ID			INT=NULL,
	@equipmentName_ID		INT=NULL,
	@equipmentType_ID		INT=NULL,
	@department_ID			INT=NULL,
	@calibrationHouse_ID	INT=NULL,
	@extensionDate			DATETIME=NULL,
	@calibrationDate		DATETIME=NULL,
	@nextCalibrationDate	DATETIME=NULL,
	@combinedUncertainty	DECIMAL(18,9)=NULL,
	@coverageFactor			DECIMAL(18,9)=NULL,
	@serialNO				VARCHAR(50)=NULL,
	@description			VARCHAR(250)=NULL,
	@traceabilityNO			VARCHAR(50)=NULL,
	@interval_ID			INT=NULL,
	@active					INT=NULL,
	@accredited				INT=NULL,
	@createdBy_ID			INT=NULL,
	@updatedBy_ID			INT=NULL,
	@approvedBy_ID			INT=NULL
AS
BEGIN

	IF EXISTS (SELECT equipment_ID FROM HEADER_Equipment WHERE equipment_ID = @equipment_ID)
		BEGIN
			UPDATE	HEADER_Equipment
			   SET	interval_ID = @interval_ID
					,department_ID = @department_ID
					,calibrationHouse_ID = @calibrationHouse_ID
					,calibrationDate = @calibrationDate
					,nextCalibrationDate = @nextCalibrationDate
					,extensionDate = @extensionDate
					,[description] = @description
					,traceabilityNO = @traceabilityNO
					,combinedUncertainty = @combinedUncertainty
					,coverageFactor = @coverageFactor
					,serialNO = @serialNO
					,active = @active
					,accredited = @accredited
					,updatedBy_ID = @updatedBy_ID
					,dateUpdated = GETDATE()
					,approvedBy_ID = @approvedBy_ID            --Receiving the Error on this line
			 WHERE	equipment_ID = @equipment_ID
			 
			 SET @returnValue = @equipment_ID
		END
	ELSE
		BEGIN
			INSERT INTO HEADER_Equipment
				(equipmentName_ID
				 ,equipmentType_ID
				 ,department_ID
				 ,calibrationHouse_ID
				 ,calibrationDate
				 ,nextCalibrationDate
				 ,extensionDate
				 ,[description]
				 ,traceabilityNO
				 ,combinedUncertainty
				 ,coverageFactor
				 ,interval_ID
				 ,serialNO
				 ,active
				 ,accredited
				 ,createdBy_ID
				 ,dateCreated
				 ,updatedBy_ID
				 ,dateUpdated
				 ,approvedBy_ID)
			 VALUES
				(@equipmentName_ID
 				 ,@equipmentType_ID
				 ,@department_ID
				 ,@calibrationDate
				 ,@nextCalibrationDate
				 ,@extensionDate
				 ,@calibrationHouse_ID
				 ,@description
				 ,@traceabilityNO
				 ,@combinedUncertainty
				 ,@coverageFactor
				 ,@interval_ID
				 ,@serialNO
				 ,@active
				 ,@accredited
				 ,@createdBy_ID
				 ,GETDATE()
				 ,@updatedBy_ID
				 ,GETDATE()
				 ,@approvedBy_ID)

			SET @returnValue = @@IDENTITY
		END	
	
	RETURN @returnValue
END

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