You'll want a formula date field that is the date halfway through the period. You'll need to decide what 'halfway' means (if there are 15 days, is 7 or 8 days halfway through the period?).
But in general - that date field would take the duration between the start and end date, divide that number by 2, then add it to the start date.
So if the duration between 2 dates is 14 days, 7 days is the halfway point. Then add 7 days to start date ([Start Date] = Days(HalfDuration).
Then use that for your summary field.
------------------------------
Mike Tamoush
------------------------------