SUMIF using Variable Columns

Hoping someone can help because I've been trying to figure this out for days!

I am looking to sum the cells only up to the column of the selected month: O12.
O12 is a drop down of all 12 months.

In the example below, the selected month is Apr-15 so in cell AZ20 I will need the sum of O20:Q20, columns prior to Apr-15.
If the month changes to May-15, I will need the sum of O20:R20 to sum in cell AZ20.

Thank you in advance for your help!

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Re: SUMIF using Variable Columns

Thank you. This formula is not allowing the revision of the sum range when I change the month in O12.

i.e
If I change the month to Mar-15 I need to sum cells O20:P20.
If I change the month to May-15 the sum range will change to O20:R20.

I am looking to sum the cells that fall BEFORE the month chosen in O12.

Re: SUMIF using Variable Columns

Quote from richadj4;784735

Off by 1 error

Just add -1 after the $O$12 reference:
=SUM($O20:INDEX($O20:$S20,MATCH($O$12-1,$O$19:$S$19)))

Worked!! Thanks a million!