Time zones and AT TIME ZONE in PostgreSQL

Introduction

Time zones help us understand what time it is in different parts of the world. The AT TIME ZONE command changes the time to a specific time zone so we can see it correctly.

You want to show the time of an event to people in different countries.

You have a meeting scheduled in UTC but want to see it in your local time.

You store times without time zone but want to convert them to a specific zone.

You want to compare times from different zones in a common format.

Syntax

PostgreSQL

timestamp_value AT TIME ZONE zone_name

timestamp_value can be a timestamp with or without time zone.

zone_name is a string like 'UTC', 'America/New_York', or an offset like '+02'.

Examples

Converts a timestamp without time zone assuming it is in local time to UTC timestamp with time zone.

PostgreSQL

TIMESTAMP '2024-06-01 12:00:00' AT TIME ZONE 'UTC'

Converts a timestamp with time zone from UTC to New York time (returns timestamp without time zone).

PostgreSQL

TIMESTAMP WITH TIME ZONE '2024-06-01 12:00:00+00' AT TIME ZONE 'America/New_York'

Converts a timestamp without time zone assuming it is in +02 offset to UTC timestamp with time zone.

PostgreSQL

TIMESTAMP '2024-06-01 12:00:00' AT TIME ZONE '+02'

Sample Program

This query shows how to convert a timestamp without time zone to UTC and New York time (both as timestamp with time zone), and how to convert a timestamp with time zone from UTC to New York time (returns timestamp without time zone).

PostgreSQL

SELECT
  TIMESTAMP '2024-06-01 12:00:00' AT TIME ZONE 'UTC' AS utc_time,
  TIMESTAMP '2024-06-01 12:00:00' AT TIME ZONE 'America/New_York' AS ny_time,
  TIMESTAMP WITH TIME ZONE '2024-06-01 12:00:00+00' AT TIME ZONE 'America/New_York' AS ny_time_from_tz;

Important Notes

When you use AT TIME ZONE on a timestamp without time zone, it treats the timestamp as if it is in the given zone and converts it to a timestamp with time zone in UTC.

When you use AT TIME ZONE on a timestamp with time zone, it converts it to the given zone but returns a timestamp without time zone.

Time zones like 'America/New_York' adjust for daylight saving time automatically.

Summary

Time zones help show correct local times worldwide.

AT TIME ZONE converts timestamps between zones.

Use it to display or compare times correctly across regions.

Practice

1. What does the AT TIME ZONE clause do in PostgreSQL?

easy

A. It converts a timestamp from one time zone to another.

B. It changes the system time zone permanently.

C. It formats the timestamp as a string.

D. It deletes time zone information from a timestamp.

Solution

1. Step 1: Understand the purpose of AT TIME ZONE

   This clause is used to convert timestamps between different time zones.

2. Step 2: Compare options

   Only It converts a timestamp from one time zone to another. correctly describes this conversion. Other options describe unrelated actions.

3. Final Answer:

   It converts a timestamp from one time zone to another. -> Option A

4. Quick Check:

   AT TIME ZONE converts time zones [OK]

Hint: Remember: AT TIME ZONE changes time zones, not system settings [OK]

Common Mistakes:

• Thinking it changes system time zone permanently
• Confusing it with formatting functions
• Assuming it removes time zone info

2. Which of the following is the correct syntax to convert a timestamp to 'America/New_York' time zone in PostgreSQL?

easy

A. SELECT timestamp_column AT TIMEZONE 'America/New_York';

B. SELECT timestamp_column AT TIME ZONE 'America/New_York';

C. SELECT timestamp_column AT TIMEZONE America/New_York;

D. SELECT timestamp_column AT TIME ZONE America/New_York;

Solution

1. Step 1: Recall correct syntax for AT TIME ZONE

   The correct syntax uses two words: AT TIME ZONE followed by the time zone in single quotes.

2. Step 2: Check each option

   Only SELECT timestamp_column AT TIME ZONE 'America/New_York'; matches the correct syntax exactly. The others either use AT TIMEZONE as one word (incorrect spelling) or miss quotes around the time zone name.

3. Final Answer:

   SELECT timestamp_column AT TIME ZONE 'America/New_York'; -> Option B

4. Quick Check:

   Use AT TIME ZONE 'zone' with quotes [OK]

Hint: Use two words: AT TIME ZONE with quotes around zone [OK]

Common Mistakes:

• Writing AT TIMEZONE as one word
• Omitting quotes around time zone
• Using incorrect keyword order

3. Given the query:

SELECT '2024-06-01 12:00:00'::timestamp AT TIME ZONE 'UTC' AT TIME ZONE 'America/Los_Angeles';

What is the output timestamp?

medium

A. 2024-06-01 20:00:00

B. 2024-06-01 04:00:00

C. 2024-06-01 05:00:00

D. 2024-06-01 12:00:00

Solution

1. Step 1: Understand the query steps

   The first AT TIME ZONE 'UTC' converts the timestamp without time zone to UTC timestamp with time zone. The second AT TIME ZONE 'America/Los_Angeles' converts that UTC time to Los Angeles local time.

2. Step 2: Calculate time difference

   On June 1, Los Angeles is on PDT (UTC-7). So 12:00 UTC minus 7 hours is 05:00 local time. But since the query applies AT TIME ZONE twice, the final output is a timestamp without time zone in Los Angeles time, which is 05:00.

3. Final Answer:

   2024-06-01 05:00:00 -> Option C

4. Quick Check:

   UTC 12:00 minus 7 hours = 05:00 [OK]

Hint: Remember LA is UTC-7 in June; subtract 7 hours [OK]

Common Mistakes:

• Confusing the order of AT TIME ZONE conversions
• Ignoring daylight saving time offset
• Assuming output is in UTC

4. Identify the error in this query:

SELECT '2024-06-01 12:00:00'::timestamp AT TIMEZONE 'Europe/London';

medium

A. The timestamp literal is missing time zone info.

B. The query is correct and will run without error.

C. The time zone name is invalid.

D. The keyword should be AT TIME ZONE with a space.

Solution

1. Step 1: Check the syntax of AT TIME ZONE

   The query uses AT TIMEZONE as one word, which is incorrect syntax in PostgreSQL.

2. Step 2: Verify other parts

   The timestamp literal is valid, and 'Europe/London' is a valid time zone name. So no error there.

3. Final Answer:

   The keyword should be AT TIME ZONE with a space. -> Option D

4. Quick Check:

   Use two words: AT TIME ZONE [OK]

Hint: Always write AT TIME ZONE as two words [OK]

Common Mistakes:

• Writing AT TIMEZONE as one word
• Assuming timestamp needs time zone info
• Thinking time zone name is invalid

5. You have a timestamp with time zone column created_at stored in UTC. You want to display it as local time in 'Asia/Tokyo' but keep it as timestamp without time zone. Which query correctly does this?

hard

A. SELECT created_at AT TIME ZONE 'Asia/Tokyo' FROM orders;

B. SELECT created_at::timestamp AT TIME ZONE 'Asia/Tokyo' FROM orders;

C. SELECT (created_at AT TIME ZONE 'UTC') AT TIME ZONE 'Asia/Tokyo' FROM orders;

D. SELECT created_at AT TIME ZONE 'UTC' AT TIME ZONE 'Asia/Tokyo' FROM orders;

Solution

1. Step 1: Understand data type and conversion

   created_at is timestamp with time zone in UTC. Applying AT TIME ZONE 'Asia/Tokyo' converts it to timestamp without time zone in Tokyo local time.

2. Step 2: Analyze options

   SELECT created_at AT TIME ZONE 'Asia/Tokyo' FROM orders; correctly applies AT TIME ZONE 'Asia/Tokyo' to convert UTC timestamptz to local timestamp. SELECT created_at::timestamp AT TIME ZONE 'Asia/Tokyo' FROM orders; casts to timestamp first, losing time zone info before conversion, causing wrong result. The options that apply AT TIME ZONE 'UTC' first perform extra and incorrect conversions.

3. Final Answer:

   SELECT created_at AT TIME ZONE 'Asia/Tokyo' FROM orders; -> Option A

4. Quick Check:

   timestamptz AT TIME ZONE zone = local timestamp [OK]

Hint: Apply AT TIME ZONE directly on timestamptz to get local timestamp [OK]

Common Mistakes:

• Casting timestamptz to timestamp before conversion
• Applying multiple AT TIME ZONE conversions unnecessarily
• Confusing timestamp with and without time zone