copy first cell to second then add hours as int (DATEPART)

Question

I have data in the table like the following.

TimeIn                   TimeOut           
-------------------------------------
6/1/2010 09:00:00   Null     
6/2/2010 09:00:00   6/2/2010 16:45:00       
6/3/2010 10:05:00   Null                    
6/4/2010 07:30:00   6/4/2010 15:45:00    

i have the stored procedure to find last activity with "not signd out" column

i have a stored procedure for copy a cell to other too ...

then, what i need, is to update [TimeOut] (if there's no time out)

via more elegant way like

UPDATE TimeOut SET DATEPART(HOUR, TimeOut) = DATEPART(HOUR, TimeIN) + 8

so the Whole idea was to first check if last activity - Timeout Column is null

then if it is, sign TimeOut with max work hours allowed (8).

is there a simple way to do it ?

UPDATE

as to marc answer , this is the selection of find out if user didn't sign out

SELECT CASE WHEN [TimeOut] IS NULL THEN '' ELSE CONVERT(NVARCHAR,[TimeOut]) END FROM tblTime WHERE tId = ( SELECT MAX(tId) FROM tblTime WHERE UserId = 123

so i have the query that finds who did not sign out at last activity

then i only need to update that specific Row - field TimeOut

with hours of time in + 8 that was my question

Answer 1

Looks like you should use dateadd.

UPDATE TimeOut 
SET TimeOut = DATEADD(HOUR, 8, TimeIN)
WHERE ....

This will set TimeOut to TimeIn plus eight hours.

Answer 2

add a where TimeOut IS NULL clause so it updates only the ones where the field actually is null?

Answer 3

UPDATE tblTime  SET TimeOut = DATEADD(HOUR,8,TimeIn) WHERE tId = ( SELECT MAX(tId) FROM tblTime WHERE UserId = 1234)