Excel 2007 : Time Formula

[AlanWade]

Hi,

I have two columns in my work sheet, one for journey start time and one for journey end time. At the moment, I have another column setup I1-H1 to give me the time to travel between jobs. Both columns are formatted to the 24 hour clock (eg 16:00). What I would like to do, if possible, is if I enter a journey start time which is before 16:00 then the calcuation will not start untill 16:00.
For example if I type start time 15:50 and end time 16:30 then the result would be 30 mins not 40.

I have enclosed an example of my work sheet so you see what mean.

Any help is appreciated.
Thanks in advance
Alan