first entry from a column and subtracts the last entry entered in the same column

Hi,
I need to write a formula that takes the first entry from a column and subtracts the last entry entered in the same column. I can't seem to do it without having to "re-set" the formula each week with the new cell number of the newest entry. I hope I was clear. I would be glad to get any advice on this. Thanks!
-S