Time related to 24hr clock

Hi all,
I am a new user of this forum and this is my first post here
I have a problem concerning having the excel to operate with 24hr times.
What i try to achieve is:
from 00:00hrs to 24:00hrs = 24hrs which is 1.0 day
Have made the attached spredsheet to calculate it correctly to 1 day.
But if you look on the attachment the "total hrs" is saying 00:00 (it transfers to 00:00 when i am putting in 24:00)
Basically how am I able to make Excel to display 24:00 insted of 00:00

Maybe there is an add ins availible like the pop up calendar but for times instead.

Any help much appreciated.
Stig