calculating times greater then 24 hours

Hi,
I'm still green with excel. I am trying to setup a spread sheat to track and calculate the amount of time my X wife spends with our kids. I set up a few columns. the first is the long date. next is pickup time, then drop off time. Drop off time is normally the next day so its down one row. The time cells are in h:mm format. I cannot calculate the difference in time. if the pick up time is 19:00 and the drop off time is 11:00, it calculates the difference as 4:00:00. Could someone please tell me what im doing wrong. It seams like such an easy thing.

Cheers