Drivers hours spreadsheet

Hello, this is probably a bit of a newbie question but I need to work out how many hours off someone has had.

I'm after a formula which looks up through the cells and for every 0 it finds it adds 24 to a total. I need this formula to stop when i finds a value that isn't 0 and adds 24hrs-that value to the total. Does that make sense?

I've attached the xls file to hopefully show what i'm trying to achieve. Column J shows the end times so and column L shows the amount of time since the person finished work.

I hope I'm making sense. Any help would be greatly appreciated.

As a side question do you all have degrees in Mathematics? Some of the things i've read are so clever and i'm in total admiration for the intelligence you all have. Thanks in advance of any help. Fess