Whats the correct formula to point to the last populated cell in a column?

[oscarlimerick]

Hi folks. The excel attachment shows a chart containing dates beside a number reported for that day. I want to be able to take the current day (March 8 in this example) and have it report the difference from the previous day (March 7th in this example). The difference is to be reported in cell K1. In this example, the cell K1 contains “=D14-D13” and correctly displays 7301, which is the difference between Mar 8 and Mar 7 # reported today.

But tomorrow, when I have a new value for cell Mar 9 in cell D15, I want cell K1 to then show the difference between Mar 9 and Mar 8. I need to find the correct formula to put in K1 that will point to the bottom most cell in column D and subtract the second last value of column D, and do this every day when there is a new entry in column D.

I realize I could just create a subtraction formula in cell E5 saying “=D5-D4” and then drop that cell box down every day so it would display the differences in column E, but I don’t want to do that every day. I just want cell K1 to constantly display the difference between the bottom most populated cell in column D (which would be the current date) and the second last bottom most populated cell in column D.(which would be yesterdays date) Is this possible? If so how can I do this? Many thanks