SUM 12 months of data not in chrono order Excel 2010

Referencing attached...I need to sum the values in column for the 12 months starting with the MOST RECENT ENTRY (in this case Jul, 2013 - August 2012). The values will be scattered, not in chrono order. The most recent date may not be the first one encountered. So I'll need to check to find the most recent date, use it as month 1 and then sum the values from month 1 through 12.

The day of the month isn't important so column K could be used to check the date.

Need it in a formula. Any suggestions?

(I'll also need to sum the values for months 13-24 but I can probably tackle that if I can get the formula for months 1-12.)
EXCEL-SAMPLE.jpg
author3000Asked:
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barry houdiniCommented:
You could use this formula for the last 12 months

=SUMIF(L2:L100,">="&EOMONTH(MAX(L2:L100),-12)+1,M2:M100)

If the max date is 7th July 2013, for example, then that will sum from August 1st 2012 onwards (assumes data in rows 2 to 100, adjust as required)

To get months 13 to 24 just subtract that figure from the figure for the last 24 months, e.g. assume above formula is in cell P2 then use this version in another cell

=SUMIF(L2:L100,">="&EOMONTH(MAX(L2:L100),-24)+1,M2:M100)-P2

regards, barry
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author3000Author Commented:
Works better than a snake charmer with a magic wand.

Thank you!
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