Formula to return percentage of duration elapsed between two dates a third date represents

Hi there!

I need a formula that will calculate the percentage of the total duration between two dates (in dd.mm.yyyy hh:mm format) that a third, intervening date represents.

As an example:

a) 10th Jan 2014 09:00
b) 20th Jan 2014 09:00
c) 30th Jan 2014 09:00

In this case the answer sould be 50% because 'b' represents exactly half of the time elapsed between 'a' and 'c'.

Note: the formula needs to work effectively across the boundary between two consecutive years.