By using the CONCATENATE formula to merge the date and time together,
you are converting it to text, and you cannot do any arithmetic with it
(which is why you are getting the #VALUE error). Use this to join the
date and time together:

=I2+J2

and format the cell as you would like it to appear. You could then just
subtract one date_time from the other to get the difference. If you
want negative times to be displayed, you will have to work in the 1904
date system.

Hope this helps.

Pete