I ran into this in a Ruby on Rails test suite a few years ago and wrote up my findings here: https://gist.github.com/abachman/f97806e1c0fe8e4e1849e5f8412...

tl;dr - ActiveSupport::TimeWithZone used absolute seconds counting to determine dates while DateTime used a proper Gregorian calendar. This means dates before October 15, 1582 have different internal values when compared.

The behavior showed up because a test was using a DateTime value to set a MySQL DATETIME column to the DB minimum value, which is 1000-01-01 00:00:00, and then comparing the DB record attribute to the original variable used to set it.

Something like:

  date = DateTime.new(1000, 1, 1, 0, 0, 0, 'UTC')
  record.update(happened_at: date)
  record.happened_at == date # => false

I "discovered" the missing Gregorian dates when I wrote a loop counting forward in time from Jan 1, 1000 until the corresponding TimeWithZone and DateTime records returned true when compared with ==.