Accounting for DST in Postgres, when selecting scheduled items

Question

I have a Postgres table of clock alarms (not really, but this is analogous, and easier to explain). Alarms are set by users with a 1 hour r

Answer 1

Given:

SET timezone = 'UTC';

CREATE TABLE tzdemo (
    username text not null,
    alarm_time_utc time not null,
    alarm_tz_abbrev text not null,
    alarm_tz text not null
);

INSERT INTO tzdemo (username, alarm_time_utc, alarm_tz_abbrev, alarm_tz) VALUES
('Alfred', TIME '04:00' AT TIME ZONE '+01:00', 'CET', 'Europe/Stockholm'),
('Lotta', TIME '05:00' AT TIME ZONE '+02:00', 'CEST', 'Europe/Stockholm'),
('Sharon', TIME '11:00' AT TIME ZONE '+08:00', 'SGT', 'Singapore');

Try:

SELECT username 
FROM tzdemo 
WHERE alarm_time_utc AT TIME ZONE alarm_tz_abbrev = TIME '03:00' AT TIME ZONE alarm_tz;

Result:

 username 
----------
 Alfred
 Sharon
(2 rows)

Principle:

• Store the timezone offset the alarm was created in including whether it was or was not DST at the time
• Also store the clock time converted to UTC
• When querying, use the fact that full timezone names follow the current UTC rules for times to produce a time that's in the current time zone for the region. Compare to the stored timestamp in what the time zone was when that alarm was created.

This also allows you to cope with cases where the user changes location, and therefore changes timezone.

This approach can be extended by date-qualifying the timestamps when you want to do predictive querying, like "at what local time will alarm sound in location".

I'm not completely confident in this solution and would recommend careful testing.

Answer 2

Use timestamp with time zone (timestamptz) for calculations.
Times for alarms can be time [without time zone].
But you have to save the time zone explicitly for every row.

Never use time with time zone It's a logically broken type, its use is discouraged by PostgreSQL. The manual:

The type time with time zone is defined by the SQL standard, but the definition exhibits properties which lead to questionable usefulness. In most cases, a combination of date, time, timestamp without timezone, and timestamp with time zone should provide a complete range of date/time functionality required by any application.

Demo setup:

CREATE TABLE alarm(name text, t time, tz text);
INSERT INTO alarm VALUES
  ('Alfred', '04:00', 'Europe/Stockholm') -- Alfred sets an alarm for 4 AM.
, ('Lotta',  '05:00', 'Europe/Stockholm') -- Lotta sets an alarm for 5 AM. 
, ('Sharon', '11:00', 'Asia/Singapore');  -- Sharon has set an alarm for 11 AM.

It has to be time zone names (not abbreviations) to account for DST. Related:

• Time zone names with identical properties yield different result when applied to timestamp

Get matching alarms for "today":

SELECT *
FROM   alarm
WHERE  (('2012-07-01'::date + t) AT TIME ZONE tz AT TIME ZONE 'UTC')::time
       = '03:00'::time

• ('2012-7-1'::date + t) ... assemble timestamp [without time zone] Could also just be now()::date + t for "today".
• AT WITH TIME ZONE tz ... place timestamp at the saved time zone, resulting in timestamptz.
• AT WITH TIME ZONE 'UTC' ... get according UTC timestamp
• ::time ... simplest way to extract the time component.

Here you can look up time zone names:

SELECT *
FROM   pg_timezone_names
WHERE  name ~~* '%sing%'
LIMIT  10

SQL Fiddle demonstrating summer / winter.

Answer 3

You would do it by using a full time zone name, e.g. America/New_York rather than EDT/EST, and storing the hour in that time zone not UTC. You can then remain blissfully ignorant of the offset changes for daylight savings.

Something like the following should work:

-- CREATE TABLE time_test (
--   user_to_alert CHARACTER VARYING (30),
--   alarm_hour TIME,
--   user_timezone CHARACTER VARYING (30)
-- );

SELECT user_to_alert, 
  CASE
    WHEN EXTRACT(HOUR FROM CURRENT_TIME AT TIME ZONE user_timezone) = EXTRACT(HOUR FROM alarm_hour) THEN TRUE
  ELSE FALSE
END AS raise_alarm
FROM time_test;

Or:

SELECT user_to_alert
FROM time_test
WHERE EXTRACT(HOUR FROM CURRENT_TIME AT TIME ZONE user_timezone) = EXTRACT(HOUR FROM alarm_hour);