Now this is obscure:  the 1904 date system in Excel.

I was working with a spreadsheet in which I was trying to create a
projection of a larger spreadsheet.  I.e., I want to be able to
share just a portion of the larger spreadsheet — so I copied the
relevant cells from the larger workbook and pasted them into the
shareable workbook as links.  Which was fine — except the dates
were all wrong.  Right day/month, wrong year.

After a few judicious Google searches, I stumbled across the fact that
the Mac-created Excel spreadsheet was using the 1904 date system as
a default; unselecting that (in preferences) restored my copied dates
to the right year.  Victory!

But, here’s the larger picture:  I need to share the projected
worksheet with staff located at remote offices.  A communications
network that lets me send data point-to-point is all I need for
that.  However, point-to-point (connection-oriented) is not appropriate to develop the sort
of widespread information network that is the World Wide Web on the
Internet. I would not know who to turn to for resolution to this
annoying glitch. (The link above is to a Microsoft site; the one that I
actually hit at the time was some random user help group). 

To state the hopefully-obvious:  we’re all thoroughly invested in
this global information service supported by the Internet, and anything
that gets in the way of J. Random Helper posting solutions that S.
Frantic User can stumble across is a significant step backwards.