Hours counting

i made calender which should cpunt how many hours i worked each day and then should add at end of the month amount from each day. I made it with format hh:mm so i can write, i came at 7:30 or something and was there until 18:00, amount of it should be then 10:30. That works good, only at end of the month i come to 180 hours or something and excel calculates it into days, means if i have f.e. 166:00 hours so far, he shows me follows "06.01.1900 22:00". My question is, How can i make him write down more then 24:00 ..
Thank you very much