Excel Calculate the date difference from today from a cell of "7/6/2012 10:26:42"

So I have a cell with 7/6/2012 10:26:42 inputted, I want to show the date difference from today in another cell.

I tried to extract 7/6/2012 with =LEFT(A1, Find(" ", A1, 1) -1) but turned out theres a value error.

The formula works when I make A1 '7/6/2012 10:26:42, however it is not ideal because I have to work with the whole column.