If your first row contains the months (real dates, not text) and if you have
a sum row for each column at the bottom (in my example it is row 11), then
this formula:
=SUMPRODUCT(--(1:1<""),--(1:1<=NOW()),11:11)
will calculate the sum of all the months which are before today's date.

Let me know in case I have misunderstood your request!

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