On Mon, 28 May 2012 16:38:33 -0700 (PDT), James Ravenswood wrote:

If I enter:
5/28/2012 in A1
and
02:15:45 in A2
then =A1 + A2 works just fine, but if I enter:
=5/28/2012 + 02:15:45 in a cell I get an error.

Why?

Because Excel does not interpret your two different types of data entry as being the same.

Entering =5/28/2012 into a cell is different than entering 5/28/2012 into a cell

The latter is interpreted as a date. The former is interpreted 0.0000887531951150241 (5 divided by 28 divided by 2012)

Similarly, entering 2:15:45 into a single cell is interpreted as a time.
Entering =2:15:45 into a cell is interpreted as an illegal range reference.

Try: ="5/28/2012" + "02:15:45"