I have a spreadsheet that I enter the current month's data
into each month. This new data is entered alongside data
entered in the previous months of the current fiscal year.

Let's say, for argument's sake, that I have just entered
the data for month three of the current fiscal year.

I want Excel to sum the data for the same three month
period during the previous fiscal year ... providing a
year-to-date comparison.

And, when I enter the data for month four of the current
fiscal year, I want Excel to sum the data for the same
four month period during the previous fiscal year ... but
I want Excel to make this transition (from three to four
months) automatically.

In other words, I don't want to rewrite the formula every
month.

Is this possible? If so, how's it done?

Thanks in advance to anyone who can help.