I am guessing that you want to cell B3 to show either the value in N3 if all three conditions are met:
A3 contains the date 31 September 2010
M3 contains the date 31 August 2010, and
L3 contains the letter "S"
This formula will do that:
To get the dates in the formula to increment by one month on each successive row, try this:
1. The following uses the EOMONTH() function. This is part of Excel's "Analysis Toolpak" which is not active by default in Excel 2003 and earlier. In Excel 2007 and later EOMONTH is part of the standard installed functions.
If you have Excel 2003 or earlier, click 'Tools' on the menu bar and select 'Add-Ins...'
From the list of Add-Ins check the box beside 'Analysis Toolpak' and click OK.
2. make the following entries on your worksheet.
In cell X1 enter 01/Aug/2010 (If X1 is in use - use another cell and substitute that cell for X1 in the following formula (but make sure to use the $ signs, so that the reference to that cell does not change as you drag the formula).
In B3 enter this formula:
This formula tests A3 for 1 day after the last day of August 2010, i.e., 01 September 2010
and it tests M3 for the last day of August 2010
Drag the formula down one row and the the formula will now test A4 for 01 October 2010
and M4 for 30 September 2010.
The formula works because the ROW() function returns the number of the row it is on, so in cell B3, ROW() returns 3. By subtracting 3 from it the End of Month function finds the end of the month zero months ahead of the date in X1, i.e., this month. On the next row, ROW() is 4 and 4-3 =1, so it returns the end of the month One month ahead of the date in X1.
Also note that entering dates as 9/1/2010 may be interpreted by Excel as 09 January, and not 01 September. It is better to be absolutely clear and use month abbreviations such as "Sep"
Hope this is of some use.