comparing DateTime instances

[Jason Orendorff]

As some icing on this topic, you can remember the
meaning of what cmp() returns like this:

  cmp(a, b) < 0   means  a < b
  cmp(a, b) == 0  means  a == b
  cmp(a, b) != 0  means  a != b
  cmp(a, b) > 0   means  a > b
  ...

Get the pattern?  Works for C's strcmp() too.

## Jason Orendorff    http://www.jorendorff.com/