Please Review This UPDATE Sql Statement For Me

Thanks for looking at this for me. Here is the SQL Script as it stands so far. 

UPDATE    TableImportHistory
SET ImportStopTime = CONVERT(numeric, GETDATE(), 108), Elapsetime = CONVERT(numeric,GETDATE(), 108) - CAST((SELECT TOP 1 ImportStartTime FROM TableImportHistory 
WHERE     TableName = 'ARCHRG'
ORDER BY DataImportID DESC) AS numeric), Successful = 1
WHERE (DataImportID = (SELECT TOP 1 DataImportID AS LastID FROM TableImportHistory WHERE TableName = 'ARCHRG') ORDER BY DataImportID DESC))

The Elapetime update portion of this statement is where I'm getting stuck (if I take it out the statement works) 

I get the error "ERROR converting Data type NVARCHAR to Numeric" I've changed the data type in the Table from numric to varchar, nvarchar, decemial nothing works and not sure what I should set the Starttime and StopTime datatypes in the table to (any suggestions please)

Elapsetime = CONVERT(numeric,GETDATE(), 108) - CAST((SELECT TOP 1 ImportStartTime FROM TableImportHistory 
WHERE     TableName = 'ARCHRG'
ORDER BY DataImportID DESC) AS numeric) 

Are there better ways to do this statement?

 

Thank you in advance

Rex