Get the last non-empty cell in a column in Google Sheets

There may be a more eloquent way, but this is the way I came up with:

The function to find the last populated cell in a column is:

=INDEX( FILTER( A:A ; NOT( ISBLANK( A:A ) ) ) ; ROWS( FILTER( A:A ; NOT( ISBLANK( A:A ) ) ) ) )

So if you combine it with your current function it would look like this:

=DAYS360(A2,INDEX( FILTER( A:A ; NOT( ISBLANK( A:A ) ) ) ; ROWS( FILTER( A:A ; NOT( ISBLANK( A:A ) ) ) ) ))

To find the last non-empty cell you can use INDEX and MATCH functions like this:

=DAYS360(A2; INDEX(A:A; MATCH(99^99;A:A; 1)))

I think this is a little bit faster and easier.

If A2:A contains dates contiguously then INDEX(A2:A,COUNT(A2:A)) will return the last date. The final formula is

=DAYS360(A2,INDEX(A2:A,COUNT(A2:A)))

My favorite is:

=INDEX(A2:A,COUNTA(A2:A),1)

So, for the OP's need:

=DAYS360(A2,INDEX(A2:A,COUNTA(A2:A),1))