Sum Data By Dates

Start Date End Date
12/1/2007 12/12/2007

Sum(datewise)

Date ID Product Qty
12/1/2007 a 2
12/1/2007 a 2
12/1/2007 c 2
12/2/2007 b 2
12/2/2007 g 2
12/2/2007 j 2
12/3/2007 i 2
12/5/2007 a 2
12/9/2007 a 2
12/12/2007 a 2
12/12/2007 d 2
12/12/2007 l 2
12/12/2007 a 2

1- What formula and macro should I use to sum from Start Date to End Date

2- What formulaand macro should I use to have a sum of particular Id from Start Date to End Date

3- How should stop formula if start date is greater than end date or not in date/number format or if both date cells are blank.