To: Leslie S Satenstein <lsatenstein@yahoo.com>
cc: Robert Haas <robertmhaas@gmail.com>,
	Postgres documentation <pgsql-docs@postgresql.org>
Subject: Re: Some comments about Julian Dates and possible bug. Please provide
	feedback.
In-reply-to: <489318.10800.qm@web110704.mail.gq1.yahoo.com> 
References: <489318.10800.qm@web110704.mail.gq1.yahoo.com>
Comments: In-reply-to Leslie S Satenstein <lsatenstein@yahoo.com>
	message dated "Wed, 29 Dec 2010 09:38:36 -0800"
Date: Wed, 29 Dec 2010 12:56:12 -0500
Message-ID: <25438.1293645372@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Leslie S Satenstein <lsatenstein@yahoo.com> writes:
>> I tested and found the algorithm in Postgres to have
>> the day before January 1,0001 calculating as December
>> 31,0000 even though the world calculates the day before
>> January 1,0001 as December 31,-0001.
> 
>> 2) Is the algorithm in Postgres correct? 狢 think
>> not, as the calculations for the difference in days between
>> January 1, 0001 and December 31,-0001 is not 367 days, but
>> just the value 1.

This is not a bug, it's just failure to understand the conventions used
internally.  If you did the calculations at the SQL level you would get
the right answers:

regression=# select '0001-01-01'::date - 1;
   ?column?    
---------------
 0001-12-31 BC
(1 row)

regression=# select '0001-01-01'::date - '0001-12-31 BC'::date;
 ?column? 
----------
        1
(1 row)

Internally we represent 1 BC as "year zero", 2 BC as "year -1", etc,
but this isn't a problem from users' perspective.

			regards, tom lane