Bibliography: item date doesn't preserve month or day are unknown

[MattHeffron]

Copied from @masinter comment on PR: Bs1 bibliography:

A date YYYY-01-01 I suspect is how it is printing dates that were just known YYYY. It would be better to not lose the distinction between 'month and day unknown' or 'month known, but day not known' and 'date really is january 01' or 'date really is YYYY-MM'