Summary: The Postgres now() function returns the current date and time as a timestamp with time zone, fixed at the start of the current transaction, making it an alias for current_timestamp and transaction_timestamp(). Use now() to auto-populate created_at or last_updated columns via DEFAULT constraints or triggers when a stable, transaction-scoped timestamp is needed. Unlike the bare now keyword (evaluated once at table creation) or clock_timestamp() (updates within a statement), now() re-evaluates on every row insert.

Get the current date and time

The Postgres now() function returns the current date and time with timezone. It's an alias for the current_timestamp() function.

This function is commonly used for timestamping database entries, calculating time differences, or implementing time-based logic in applications. For instance, you might use it to record when a user creates an account, when an order is placed, or to calculate intervals - like how long ago an event occurred.

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The now() function has a single form:

now() -> timestamp with timezone

This form returns the current timestamp with the timezone at the start of the current transaction.

Let's consider a user_accounts table that tracks user registration information. We can use now() to record the exact time a user creates their account.

CREATE TABLE user_accounts (
  user_id SERIAL PRIMARY KEY,
  username VARCHAR(50) UNIQUE NOT NULL,
  email VARCHAR(100) UNIQUE NOT NULL,
  created_at TIMESTAMP WITH TIME ZONE DEFAULT now()
);

INSERT INTO user_accounts (username, email)
VALUES ('john_doe', 'john@example.com');

This query creates a table to store user account information, with the created_at column automatically set to the current timestamp when a new record is inserted.

Let's insert another record and retrieve all user accounts:

INSERT INTO user_accounts (username, email)
VALUES ('jane_smith', 'jane@example.com');

SELECT * FROM user_accounts;

This query returns the following output:

 user_id |  username  |      email       |          created_at
---------+------------+------------------+-------------------------------
       1 | john_doe   | john@example.com | 2024-06-25 08:40:25.603165+00
       2 | jane_smith | jane@example.com | 2024-06-25 08:40:38.220631+00
(2 rows)

We can use now() in combination with stored timestamps to calculate time differences. For example, let's create a table to track project deadlines and calculate how much time is left:

CREATE TABLE projects (
  project_id SERIAL PRIMARY KEY,
  project_name VARCHAR(100) NOT NULL,
  start_date TIMESTAMP WITH TIME ZONE DEFAULT now(),
  deadline TIMESTAMP WITH TIME ZONE NOT NULL
);

INSERT INTO projects (project_name, deadline)
VALUES
  ('Website Redesign', now() + INTERVAL '30 days'),
  ('Mobile App Development', now() + INTERVAL '60 days'),
  ('Database Migration', now() + INTERVAL '15 days');

SELECT
  project_name,
  deadline - now() AS time_remaining
FROM projects
ORDER BY time_remaining;

This query calculates and displays the remaining time for each project, ordered from the most to the least urgent.

      project_name      |     time_remaining
------------------------+------------------------
 Database Migration     | 14 days 23:59:59.93332
 Website Redesign       | 29 days 23:59:59.93332
 Mobile App Development | 59 days 23:59:59.93332
(3 rows)

We can use now() in combination with an update trigger to automatically maintain modification timestamps for records.

Here's an example using a table for tracking customer orders. It has columns for both the creation and last update timestamps, with a trigger that updates the last_updated column whenever an order is modified:

CREATE TABLE customer_orders (
  order_id SERIAL PRIMARY KEY,
  customer_id INTEGER NOT NULL,
  order_status VARCHAR(20) NOT NULL,
  created_at TIMESTAMP WITH TIME ZONE DEFAULT now(),
  last_updated TIMESTAMP WITH TIME ZONE DEFAULT now()
);

CREATE OR REPLACE FUNCTION update_last_updated_column()
RETURNS TRIGGER AS $$
BEGIN
    NEW.last_updated = now();
    RETURN NEW;
END;
$$ language 'plpgsql';

CREATE TRIGGER update_customer_order_timestamp
BEFORE UPDATE ON customer_orders
FOR EACH ROW
EXECUTE FUNCTION update_last_updated_column();

INSERT INTO customer_orders (customer_id, order_status)
VALUES (1001, 'Pending'), (1002, 'Processing');

Now, let's update an order and observe the changes:

-- Simulate some delay before update
SELECT pg_sleep(2);
UPDATE customer_orders SET order_status = 'Shipped' WHERE order_id = 1;

SELECT * FROM customer_orders;

This query returns the following output, showing the updated status and the new last_updated timestamp, for the modified order.

 order_id | customer_id | order_status |          created_at          |         last_updated
----------+-------------+--------------+------------------------------+-------------------------------
        2 |        1002 | Processing   | 2024-06-25 09:26:43.57742+00 | 2024-06-25 09:26:43.57742+00
        1 |        1001 | Shipped      | 2024-06-25 09:26:43.57742+00 | 2024-06-25 09:26:50.962194+00
(2 rows)

We can wrap now() in a user-defined function to perform more complex date/time calculations. For example, here's a function that calculates the current age of a user.

CREATE OR REPLACE FUNCTION calculate_age(birth_date DATE)
RETURNS INTEGER AS $$
BEGIN
    RETURN DATE_PART('year', AGE(now(), birth_date));
END;
$$ LANGUAGE plpgsql;

SELECT
  calculate_age('1990-05-15') AS age_1,
  calculate_age('2000-12-31') AS age_2,
  calculate_age('1985-03-20') AS age_3;

This query calculates the age of three users based on their date of birth:

 age_1 | age_2 | age_3
-------+-------+-------
    34 |    23 |    39
(1 row)

Like current_timestamp, now() returns a value in the timezone of the current session. This defaults to the server's timezone unless explicitly set in the session. It's important to keep this in mind when working with timestamps across different timezones.

The now() function is a built-in function that returns the current timestamp with the timezone. In contrast, the keyword now (without parentheses) is a reserved word that is converted to the current timestamp value when first parsed.

It is recommended to use now() for clarity and consistency. For example, if the default value for a column is set to now, it will be evaluated once when the table is created and reused for all successive records. Whereas, now() will be evaluated each time a new row is inserted, which is the typically desired behavior.

• current_timestamp() - Functionally identical to now().
• transaction_timestamp() - Returns the current timestamp at the start of the current transaction, also equivalent to now().
• statement_timestamp() - Returns the current timestamp at the start of the current statement.
• clock_timestamp() - Returns the actual current timestamp with timezone, which can change even during a single SQL statement.

• PostgreSQL documentation: Date/Time Functions and Operators
• PostgreSQL documentation: Date/Time Types

• age
• current_timestamp
• date_trunc
• extract

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