Time zone with daylight savings times in PostgreSQL

Question

We're deploying our own stream gauges (a lot like this USGS gauge: http://waterdata.usgs.gov/usa/nwis/uv?site_no=03539600) so us kayakers know whether or not there's enough water to paddle the stream and don't waste time and gas to drive out there. We hope install a few of these across the southeast whitewater region which spans the eastern and central time zones.

I'm storing the time a record is inserted using the default value of current_time for the record. I'd like to later display the data using the MM/DD/YYYY HH12:MI AM TZ format, which outputs reading like 03/12/2012 01:00 AM CDT. I'd also like for the output to be aware of changes in day light savings time, so the last part of the previous sentence would change between CST and CDT when we 'spring forward' and 'fall back'. This change occurred on 3/11/2012 this year and I've included dates on both sides of this DST line below. I'm using my Windows 7 laptop for development and we will later be deploying on a Unix box. Postgres has apparently detected that my Windows computer is set to eastern US time zone. I'm trying this with a 'timestamp without time zone' field and a 'timestamp with time zone' field but can't get it to work.

I've tried using 'at time zone' in my selects and every thing is working until it's time to display the time zone. The actual hour is part of the time stamp is correctly subtracted by an hour when I ask for the time in CDT. But EDT is displayed in the output.

SELECT reading_time as raw,
       reading_time at time zone 'CDT',
       to_char(reading_time at time zone 'CDT',
           'MM/DD/YYYY HH12:MI AM TZ') as formatted_time
  FROM readings2;

"2012-04-29 17:59:35.65";"2012-04-29 18:59:35.65-04";"04/29/2012 06:59 PM EDT"
"2012-04-29 17:59:40.19";"2012-04-29 18:59:40.19-04";"04/29/2012 06:59 PM EDT"
"2012-03-10 00:00:00";"2012-03-10 00:00:00-05";"03/10/2012 12:00 AM EST"
"2012-03-11 00:00:00";"2012-03-11 00:00:00-05";"03/11/2012 12:00 AM EST"
"2012-03-12 00:00:00";"2012-03-12 01:00:00-04";"03/12/2012 01:00 AM EDT"

I'm storing the time zone that each of our gauges is located in a character varying field a separate table. I considered just appending this value to the end of the time output, but I want it to change from from CST to CDT without my intervention.

Thanks for your help.

Answer 1

Instead of using time zone names like CDT or CST, you could consider using full Olsen-style time zone names. In the case of central time, you could choose a time zone. Either one that matches your location, such as America/Chicago, or just US/Central. This ensures PostgreSQL uses the Olsen tz database to automatically figure out whether daylight saving time applies at any given date.

Answer 2

You definitely want a TIMESTAMP WITH TIME ZONE column (which is also known as timestamptz in PostgreSQL). That will store the timestamp in UTC, so that it represents a particular moment in time. Contrary to what the name suggests, it does not save a time zone in the column -- you can view the retrieved timestamp in the time zone of your choosing with the AT TIME ZONE phrase.

The semantics of TIMESTAMP WITHOUT TIME ZONE are confusing and nearly useless. I strongly recommend you don't use that type at all for what you are describing.

I'm really confused by the part of the question which talks about storing the timestamp in a CHARACTER VARYING column. That seems as though it might be part of the problem. If you can store it in timestamptz right from the start I suspect that you will have fewer problems. Barring that, it would be safest to use the -04 notation for offset from UTC; but that seems like more work to me for no benefit.

Answer 3

You can create a table of known timezones in the format suggested in Guan Yang's answer, and then use a foreign key column to this table. Valid timezones can be obtained from pg_timezone_names I've gone into more detail in this related answer.