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

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);