Thread: "Julian day" date format is off by 12 hours
Postgres version: 8.0.6
Operating system: Ubuntu GNU/Linux
I executed the following query while trying to build some date-conversion
functions for data that was represented as milliseconds since the Unix epoch:
davidl=# SELECT to_char(timestamp '1970-01-01 00:00:00 GMT','J SSSS MS');
to_char
---------------
2440588 0 000
(1 row)
However, Postgres's notion of a "Julian Day" does not match the
generally-accepted definition. According to the generally-accepted
definition, the result of the query above should be
2440587 43200 000
; that is, 12 hours past noon on Julian day 2440687, which started at noon
on December 31st, 1969, GMT.
I'm not sure if this should be regarded as a database bug or a documentation
bug. Table 9-21 in the manual only says that a Julian day is "days since
January 1, 4712 BC", so Postgres is consistent with the manual; but every
other definition of a Julian day I've found says that it starts at noon.
The Wikipedia article has several good references:
http://en.wikipedia.org/wiki/Julian_day
--
Software Developer, Precision Motor Transport Group, LLC
Work phone 517-349-3011 x215
Cell phone 586-873-8813
Since to_char() is supposed to be Oracle-compatible, would someone test this query in Oracle? --------------------------------------------------------------------------- David Lee Lambert wrote: > Postgres version: 8.0.6 > Operating system: Ubuntu GNU/Linux > > I executed the following query while trying to build some date-conversion > functions for data that was represented as milliseconds since the Unix epoch: > > davidl=# SELECT to_char(timestamp '1970-01-01 00:00:00 GMT','J SSSS MS'); > to_char > --------------- > 2440588 0 000 > (1 row) > > However, Postgres's notion of a "Julian Day" does not match the > generally-accepted definition. According to the generally-accepted > definition, the result of the query above should be > > 2440587 43200 000 > > ; that is, 12 hours past noon on Julian day 2440687, which started at noon > on December 31st, 1969, GMT. > > I'm not sure if this should be regarded as a database bug or a documentation > bug. Table 9-21 in the manual only says that a Julian day is "days since > January 1, 4712 BC", so Postgres is consistent with the manual; but every > other definition of a Julian day I've found says that it starts at noon. > > The Wikipedia article has several good references: > > http://en.wikipedia.org/wiki/Julian_day > > -- > > Software Developer, Precision Motor Transport Group, LLC > Work phone 517-349-3011 x215 > Cell phone 586-873-8813 > > ---------------------------(end of broadcast)--------------------------- > TIP 3: Have you checked our extensive FAQ? > > http://www.postgresql.org/docs/faq -- Bruce Momjian bruce@momjian.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. +
Oracle (9.2.0.7) doesn't seem to like the date format string...
SQL> SELECT to_char(timestamp '1970-01-01 00:00:00 GMT','J SSSS MS') from d=
ual;
SELECT to_char(timestamp '1970-01-01 00:00:00 GMT','J SSSS MS') from dual
*
ERROR at line 1:
ORA-01821: date format not recognized
Regards,
Shelby Cain
----- Original Message ----
From: Bruce Momjian <bruce@momjian.us>
To: David Lee Lambert <dlambert@bmtcarhaul.com>
Cc: pgsql-bugs@postgresql.org
Sent: Saturday, February 3, 2007 5:56:00 PM
Subject: Re: [BUGS] "Julian day" date format is off by 12 hours
Since to_char() is supposed to be Oracle-compatible, would someone test
this query in Oracle?
>=20
> davidl=3D# SELECT to_char(timestamp '1970-01-01 00:00:00 GMT','J SSSS MS'=
);
> to_char
> ---------------
> 2440588 0 000
> (1 row)
>=20
=20
___________________________________________________________________________=
_________
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I did some research on this, and because Oracle fails with that query, we can't use them as a guide. What is happening in the code is that the "J" is independent from the "SSSS" and "MS", so you are getting a "J" based on the date (assuming midnight start/stop), and not on the actual time in the rest of the timestamp. As you stated, to do this correctly 11:59am would have a different Julian date from 12:01pm. However, I think this would make "J" much less useful because the most common use assumes midnight to 11:59pm is the same day number. What I did was to update the documentation to say explicitly "midnight": Julian Day (days since midnight, January 1, 4712 BC) I didn't document that we don't follow the specification, but the "midnight" should be a hint for those who know about it. --------------------------------------------------------------------------- bruce wrote: > > Since to_char() is supposed to be Oracle-compatible, would someone test > this query in Oracle? > > --------------------------------------------------------------------------- > > David Lee Lambert wrote: > > Postgres version: 8.0.6 > > Operating system: Ubuntu GNU/Linux > > > > I executed the following query while trying to build some date-conversion > > functions for data that was represented as milliseconds since the Unix epoch: > > > > davidl=# SELECT to_char(timestamp '1970-01-01 00:00:00 GMT','J SSSS MS'); > > to_char > > --------------- > > 2440588 0 000 > > (1 row) > > > > However, Postgres's notion of a "Julian Day" does not match the > > generally-accepted definition. According to the generally-accepted > > definition, the result of the query above should be > > > > 2440587 43200 000 > > > > ; that is, 12 hours past noon on Julian day 2440687, which started at noon > > on December 31st, 1969, GMT. > > > > I'm not sure if this should be regarded as a database bug or a documentation > > bug. Table 9-21 in the manual only says that a Julian day is "days since > > January 1, 4712 BC", so Postgres is consistent with the manual; but every > > other definition of a Julian day I've found says that it starts at noon. > > > > The Wikipedia article has several good references: > > > > http://en.wikipedia.org/wiki/Julian_day > > > > -- > > > > Software Developer, Precision Motor Transport Group, LLC > > Work phone 517-349-3011 x215 > > Cell phone 586-873-8813 > > > > ---------------------------(end of broadcast)--------------------------- > > TIP 3: Have you checked our extensive FAQ? > > > > http://www.postgresql.org/docs/faq > > -- > Bruce Momjian bruce@momjian.us > EnterpriseDB http://www.enterprisedb.com > > + If your life is a hard drive, Christ can be your backup. + -- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. +