In Ruby trunk date changed to use the gregorian calendar in
https://github.com/ruby/date/pull/73.

This had the result that dates not in the gregorian calendar can't be
compared to times not in the gregorian calendar. Since this test isn't
testing datetime comparison but rather that bc timestamps can be stored
in the database, we can compare with `Time.new(0) - 1.week` rather than
`Date.new(0) - 1.week`. I think this might be a bug in Ruby but I'm not
sure there's a great fix. For now however, let's get this test passing.