Hi,

I had to do something similar, this was the fastest technique:

Let A & B be your quarter start and end date, X & Y the period covered on
the electricity bill. You want to work out the overlap between these two
periods (you can do the same for the next quarter after AB etc)

The biggest the overlap can be is: A+1-B days (if A and B are the same it's
1 day, hence the +1) So, so far =(A+1-B)

If the bill period ends before the quarter period then you need to knock
theses days off. This is Max(B-Y,0), so we have =(A+1-B) - Max(B-Y,0)

If the bill period starts after the quarter period then you need to knock
these days off, this is Max(X-A,0)... This gives
=(A+1-B)-Max(B-Y,0)-Max(X-A,0)

As a final point, the whole lot must be at least 0 days, so this must be the
final formula:

=Max((A+1-B)-Max(B-Y,0)-Max(X-A,0),0)

I hope that makes sense...