there are more ways than one when it comes to accomplishing stuff in excel. I personally would combine the index function with average, like this:

=AVERAGE(A1:INDEX(A1:L1,$T$1))

where your data for the year is in A1 thru L1, and far off to the right you have cell T1, in which you enter a number between 1 and 12 to indicate how far along the range A1:L1 you want to use for your average. This way if I want to look at the YTD average for a prior period I can manipulate it, rather than relying on the system date.

I have the $ in there because this is a personal finance forum. JK. It's because in the spreadsheet I use this in, I am calculating the average the same way on multiple rows, so I if I copy the formula down, the index function argument always refers to cell T1, no matter what row I am copying to. If I copy the formula to the row below, it would read =AVERAGE(A2:INDEX(A2:L2,$T$1)). Sorry if you already knew that.