[xsl] format-date() and negative (BCE) dates

["Martin Holmes gtxxgm-xsl-list-2@xxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Consider this BCE date:

  -0001-12-29

If I format this using the "AD" (Gregorian) calendar, like this:

  format-date(xs:date('-0001-12-29'),
    '[Y0001]-[M01]-[D01]', (), 'AD', ())

I get an invalid date:

  0000-12-29

There is no year zero in the Gregorian calendar. I get the same results 
using OS (Julian). What am I doing wrong here? The ISO calendar does 
have a year zero. If I include the E (Era) component in the picture 
string, I do get "BC", but I don't see why the correct negative-one year 
is being changed to an incorrect zero-year.

This is using Saxon 9.4 (the one distributed with Oxygen 15.2).

All help appreciated,
Martin