How can I calculate the difference between two time fields, being the second greatest than the first one?

I'm having trouble getting negative hour as a result. For instance:

G           H08:30       06:00

I would like to get a column with the difference between H and G. So I would be filled with -02:30.

When I do a simple calculation, like G2-H2, I get 22:00 as a result.

Any ideas on how to fix this?

I saw this post How to handle Negative Time Delta in Google Spreadsheets, but none of them have the solution I'm looking for.