Summing columns depending on date selected

Country Total Jun-12 Jul-12 Aug-12
Australia 12 4 4 4

Hi guys

I have the above table in Excel, and I want to sum the month columns depending on which month I have selected in the report (always starting from the Jun-12 column), which go in the total column.

So, in the total column, if I have Aug-12 selected in another cell, I want the total to sum Jun, Jul and Aug, but if I have Jul-12 selected, just sum the values in Jun and Jul.

Is this possible?

I have done a similar thing using match for ranges, but not sure how this is applied to normal forumlae?

Thanks,
John

Try:

=SUM($B2:INDEX($B2:$Z2,MATCH($A$10,$B$1:$Z$1,0))

where first row of values is in B2:Z2, with dates in B1:Z1, and the input date is in A10.