Does PostgreSQL take into account daylight saving time in its calendar?
For the last summer hour of the spring (Daylight Saving Time), on Sunday March 27, 2022:
SELECT to_timestamp('20220329 00:00:00','yyyymmdd hh24:mi:ss') - to_timestamp('20220320 00:00:00','yyyymmdd hh24:mi:ss') intervalle ;
intervalle
------------
9 days
Regards
----- Mail original -----
De: "Laurenz Albe" <laurenz.albe@xxxxxxxxxxx>
À: "PALAYRET Jacques" <jacques.palayret@xxxxxxxx>, pgsql-general@xxxxxxxxxxxxxx
Envoyé: Lundi 27 Février 2023 09:23:37
Objet: Re: Interval in hours but not in days Leap second not taken into account
On Mon, 2023-02-27 at 07:26 +0000, PALAYRET Jacques wrote:
> # An interval in " years months ... seconds " given in seconds by EXTRACT(EPOCH ...) transtyped into INTERVAL :
> SELECT (EXTRACT(EPOCH FROM ('3 years 2 months 1 day 10 hours 11 minutes 12 seconds'::interval) ) || ' seconds')::interval ;
> interval
> -------------
> 27772:11:12
>
> # The same interval in seconds formated with TO_CHAR() :
> SELECT TO_CHAR((EXTRACT(EPOCH FROM ('3 years 2 months 1 day 10 hours 11 minutes 12 seconds'::interval) ) || ' seconds')::interval, ' yyyy mm dd_ hh24-mi-ss ') ;
> to_char
> ---------------------------
> 0000 00 00_ 27754-11-12
>
> => The result is given in hours ... (not in days ...).
>
> It is logical that there are neither years nor months because they are not constant
> (leap year or not; a month can contain 31 30 ... days).
> I thought that days were eliminated because of the leap second (extra seconds
> inserted in the UTC time scale); obviously, this is not the case.
>
> # PostgreSQL does not take into account the additional second (leap second) in some calendar days ; eg. 2016, 31 dec. :
> SELECT to_timestamp('20170102 10:11:12','yyyymmdd hh24:mi:ss') - to_timestamp('20161230 00:00:00','yyyymmdd hh24:mi:ss') intervalle ;
> intervalle
> -----------------
> 3 days 10:11:12
> --> With postgreSQL, a calendar day is always 86,400 seconds long.
>
> So, is there a reason for this (interval in hours ...) ?
The best explanation I have is "daylight savings time".
One day is not always 24 hours long.
If you keep the interval in hours, the result is always correct (if you
ignore leap seconds, which PostgreSQL doesn't account for).
Yours,
Laurenz Albe
