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Subject: Re: Timestamp Shifts when calling to_timestamp(extract (epoch from
 timestamp))
From: Scott Ribe <scott_ribe@elevated-dev.com>
In-Reply-To: 
 <CACT-NGKQRtBGKOGUMT_goxyKu=yym8ET1Q5my-yjfuWW++uXbA@mail.gmail.com>
Date: Tue, 12 May 2015 11:53:23 -0600
Cc: "pgsql-admin@postgresql.org" <pgsql-admin@postgresql.org>
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Message-Id: <4FD9A31F-C7E3-4A27-B431-5B284BD3EF28@elevated-dev.com>
References: 
 <CACT-NGKQRtBGKOGUMT_goxyKu=yym8ET1Q5my-yjfuWW++uXbA@mail.gmail.com>
To: Robert Burgholzer <rburghol@vt.edu>
Precedence: bulk
Sender: pgsql-admin-owner@postgresql.org

On May 12, 2015, at 11:33 AM, Robert Burgholzer <rburghol@vt.edu> wrote:
>=20
> In postgresql 9.3 I am running into what I consider counterintuitive beha=
vior when I convert something to a Unix epoch, then back from a timestamp w=
ithout timezone.  Calling "to_timestamp(extract (epoch from timestamp))" re=
turns a time that is shifted the distance from local time to GMT (Example 1=
).  I have a workaround for when I do data imports, in that if I create col=
umns as "timestamp with timezone" and do the same conversion, they convert =
to and fro seemelessly (example 2).=20=20
>=20
> Thoughts on this?  To me, it would seem intuitive that if you did not spe=
cify a timezone, the db would choose it's own local timestamp as the timezo=
ne.

=46rom the documentation on date/time data types:

"Note: The SQL standard requires that writing just timestamp be equivalent =
to timestamp without time zone, and PostgreSQL honors that behavior. timest=
amptz is accepted as an abbreviation for timestamp with time zone; this is =
a PostgreSQL extension.=E2=80=9D

Then from date/time functions:

"epoch
For timestamp with time zone values, the number of seconds since 1970-01-01=
 00:00:00 UTC (can be negative); for date andtimestamp values, the number o=
f seconds since 1970-01-01 00:00:00 local time; for interval values, the to=
tal number of seconds in the interval=E2=80=9D

So you get number of seconds from UTC your local time in the call to epoch,=
 essentially ignoring the specified =E2=80=98EST=E2=80=99 time zone. Then o=
n converting back, it=E2=80=99s treated as seconds from UTC.

--=20
Scott Ribe
scott_ribe@elevated-dev.com
http://www.elevated-dev.com/
https://www.linkedin.com/in/scottribe/
(303) 722-0567 voice


--=20
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