Summing a number of columns depending on which period I am in.

I have a spreadsheet setting out monthly information. Each month the period changes and I am required to change the current formula to include the addition of a further column to give me a year to date figure. For instance if my periods 1 to 6 were in clolums A,B,C,D,E and F and I am currently in period 5 my formula is currently =SUM(A?:E?) however upon moving into period 6 I would then change the formula to =SUM(A?:F?) and then copy this formula all the way down. Is there any formula that I could use to add up the required columns depending on the period I am in without the need to remember to change the formula and then copy this formula down?
I'm in Excel 2007