Okay, once and for all, I really need to learn how to deal with zeros.
Whenever I've had a problem, if I can't phase it out with conditional
formatting and coloured cells, I've asked a couple of times or so for
help on individual cases to get rid of the zeros properly through the
formula. But I've looked through various examples and I can't see the
pattern. So perhaps some kind soul can give me a general overview?
Perhaps my brain will register once and for all how to deal with those
pesky zeros that come up due to results of a formula based on, as yet,
empty source fields?

In this good example of what I'm trying to fix today, it's a simple
formula:

=SUM('2007-2009'!L22:O22)

When the source sheet has no data in it in the pertinent cells, then
the target sheet where this formula is in shows a "0". We need to
keep the cells truly blank in those cases.

Thanks! :oD