Implementation of Ord for Date assumes equal Calendars

[AngusGMorrison]

The implementation of Ord for Date assumes that the Calendars of the inputs date are the same.

This isn't a terrible assumption, since the Date constructor doesn't currently allow the creation of overlapping dates in different calendars, and Dates are immutable. Still, we shouldn't lose sight of this as the API evolves.

To do this properly, in the situation where we're comparing a Gregorian date with a date in either of the Julian calendars, we'd need a function to check whether the Julian calendar date is 14 days or less ahead of a Gregorian date, and adjust the return value of Ord accordingly.