Please can anyone point me to the best way to do this, I expect it's a
common problem and my head is spinning trying to work out the simple formula
needed:

I have a sheet with rows showing tasks in a project, each task has
columns showing a start & a finish date. Other columns show months.
I need to show how many days of each task occur in each month, so I can
bill for time spent each month.

eg, as in a gantt bar chart:
task start finish Jan Feb March
design 1 jan 31 Jan 31 0 0
develop 1 jan 20 Feb 31 20 0
etc.

It must be something like "startofmonth-taskstart..........etc" but there
seem to be 6 different combinations of how a task overlaps the month you
want, and I'm looking for a simple calculation, not 6 nested Ifs!

Thanks if anyone can help.