How can I calculate the duration of sleep in emacs table?

I have a table containing Date, Waketime and Sleeptime as dates. Based on these data, I want to calculate the duration of sleep the night before. Below you can see an example of a table that I would like to get:

| Date             | Waketime | Bedtime | Sleep |
| <2012-09-24 Mon> |     6:00 |   22:00 |       |
| <2012-09-25 Tue> |     8:00 |   01:00 | 10:00 |
| <2012-09-26 Wed> |     7:00 |   23:00 |  6:00 |

When I apply the table formula:, #+TBLFM: $4=if((24-$3)<12, (24-$3)+@-1$2, $3+@-1$2);tthis does not give the desired result due to the fact that integers (24, 12 ...) are interpreted as seconds.

Invalid result:

| Date             | Waketime | Bedtime |  Sleep |
| <2012-09-24 Mon> |     6:00 |   22:00 |        |
| <2012-09-25 Tue> |     8:00 |   01:00 |   5.01 |
| <2012-09-26 Wed> |     7:00 |   23:00 | -14.99 |

   #+TBLFM: $4=if((24-$3)<12, (24-$3)+@-1$2, $3+@-1$2);t

How can I adjust this formula so that it returns the correct duration of sleep?

, if, -, Sleeptime. 12 , Bedtime Waketime. 12 , 24 Bedtime Waketime