Get the timestamp of the last and first day of a month in postgresSQL

Question

I am currently trying to figure out the equivalent functions in PostgresSQL for the below MariaDB functions:

SELECT
CONCAT(DATE_ADD(DATE_ADD(LAST_DAY('2021-02-15 00:00:00'),INTERVAL 1 DAY),INTERVAL -1 MONTH), ' ', '00:00:00') AS first_day_of_the_month,
CONCAT(LAST_DAY('2021-02-15 23:59:59'), ' ' ,'23:59:59') AS last_day_of_the_month;

Expected Result:

first_day_of_the_month  |    last_day_of_the_month
------------------------|-------------------------------
2021-02-01 00:00:00     |    2021-02-28 23:59:59

---

I want to get the last day and the first day of the month of a certain date including the time 00:00:00 for the first day and 23:549:59 for the last day.

So far I am comming close to the expected result with these queries:

SELECT 

(date_trunc('MONTH', '2021-02-15 00:00:00'::TIMESTAMP) + INTERVAL '1 MONTH - 1 day')::TIMESTAMP
+ '1 days'::INTERVAL - '1 months'::INTERVAL AS first_day_of_the_month,

(date_trunc('MONTH', '2021-02-15 23:59:59'::TIMESTAMP) + INTERVAL '1 MONTH - 1 day')::TIMESTAMP AS last_day_of_the_month;

However, as you can see in the DB-Fiddle for the last_day_of_the_month I am getting 00:00:00 instead of 23:59:59 .

How do I need to change the query to get the correct timestamp?

Answer 1

For the first day of the month:

date_trunc('month', the_date_column)

For the last day:

date_trunc('month', the_date_column) + interval '1 month' - interval '1 day'

Or if you prefer:

date_trunc('month', date) + interval '1 month - 1 day'

If you really one one second from midnight on the last day, replace day with second in the logic. I don't recommend that. In fact, if you want to define a range of date, define an open range and just use the first day of the next month. Then you can learn about tsrange which builds this into the language.

Answer 2

demo:db<>fiddle

/* Option 1: Certain Date */
SELECT
date_trunc('month', timestamp '2021-02-15 00:00:00') AS first_timestamp,
date_trunc('month', timestamp '2021-02-15 23:59:59') + interval '1 month - 1 second' AS last_timestamp;

/* Option 2: Current_date */
SELECT
date_trunc('month', current_date)::timestamp AS first_timestamp,
date_trunc('month', current_date)::timestamp + interval '1 month - 1 second' AS last_timestamp;

First timestamp of month

date_trunc('month', ...) normalizes the date to the first possible timestamp of the month. So it returns 00:00:00 of the first day of the month

Last timestamp of month

If you add a month to the previous result, you get the first day/timestamp ( 00:00:00 ) of the next month. If you subtract a day from this, you get the last day of the current month. Of course, you can subtract just a second to get 23:59:59 of the last day of the month as you expected.