Add or subtract times in an org-table

[Bastian Bechtold]

Hi,

I am an enthusiastic user of org-mode for organizing my work and
clocking my work time.

At the end of each week, I use org-agenda to view a summary of what I
worked on that week and how long I worked per project and in total. This
is great!

Then, I put this into an org-table that sums up the time I worked
against my quota like this:

| week |  time | quota |  diff | overtime | comments               |
|------+-------+-------+-------+----------+------------------------|
| W01  | 11:45 | 12:00 | -0:15 |    -0:15 |                        |
| W02  | 14:30 | 12:00 |  1:30 |     1:15 |                        |
| W03  |  9:00 |  9:36 | -0:36 |     0:39 | sick on friday (-2:24) |

Where the second column is the time I actually worked, the third column
is the time I should have worked, the fourth column is the difference
between the second and the third (i.e. overtime), and the fifth column
sums up the overtime over time.

The thing is, I do all this by hand because I can't figure out how to
add or subtract times in emacs.

Could any of you emacs wizards help me with that? Any hints would be
appreciated!

Kind regards,
Bastian