Hi again,
If you can squeeze your data into two columns (one for date and one for the relevant data) I have a solution!
Use SUMIFS. Put all dates in column A and all the data in column B in one worksheet, let's call it MyData and let's pretend you have 10 rows of data (doesn't matter if some rows are some other text, those will just be skipped).
Then open another worksheet and write your criteria in column A and B. Let's say you want to sum all data from dates 17 september to 23 september. Write
>=2012-19-17 in column A
<=2012-09-23 in column B (or whatever date format you prefer)
Then you can write in column C SUMIFS(MyData(B1:B10); condition 1(reference to column A on this row); MyData(A1:A10); condition 2(reference to column B on this row); MyData(A1:A10))
This will sum all numbers concearning those dates!

If you have english Excel all ";" above should be "," (I have swedish Excel :-( ) And all the references should be written by marking in your Excel sheet, I can't write the exact syntax now since I'm not at work by my sheet.

All the best,
Helena