Date/Time Types : internals

Hi,

At the bottom of the page about Date/Time types (
http://www.postgresql.org/docs/9.1/interactive/datatype-datetime.html
)
there is this sentence :

Date conventions before the 19th century make for interesting reading,
but are not consistent enough to warrant coding into a date/time handler.


This sentence seemed very strange to me, and I am not sure to really
understand what it implies (or not) for the user. Could someone explain
that this really means and implies?

Thank you,

Florence Cousin.