If Statement Using Offset Values For Dates

I'm trying to write code so that it uses an offset value depending on criteria. In the attached book I have three coloumns, firstly the date, secondly some letters, an If statement in the third coloumn and another IF statements in the fourth coloumn. I would like a macro that calculates the number of DAYS ONLY, depending on the fourth coloumn. The way it should operate is it should is detect a 1 in the D coloumn and then calculate the difference in days from the adjacent date in coloumn A and subtract from the first time it appears in coloumn A.

For instance in the worksheet, the first 1 in coloumn D is in 'D5'. The date in 'A5' is 04/01/2005. The first time a '1' appears in coloumn C is C2 and its date in 'A2' is 01/01/2005, so 04/01/2005-01/01/2005 is 3 days. Now heres the twist, the next 1 in the D coloumn is in 'D8' and hence the date in A8 is 07/01/2005, now this time the '1' in coloumn C is in 'C7' and hence the new calculation is 07/01/2005-06/01/2005. So hence the 1 in coloumn C is always changing (ie new offset value).