MySQL: “FROM_UNIXTIME(0)” gives “1970-01-01 01:00:00”, which is 1 hour off

datetimeMySQL

Context: MySQL version

On MySQL 5.7.17 (as shown by SHOW VARIABLES LIKE "%version%";)

Context: MySQL timezone settings

and with SELECT @@global.time_zone, @@session.time_zone; yielding

+--------------------+---------------------+
| @@global.time_zone | @@session.time_zone |
+--------------------+---------------------+
| SYSTEM             | SYSTEM              |
+--------------------+---------------------+

…which means the server tracks the system timezone.

Context: System timezone

This being a Linux, we check the system timezone like this (output redacted):

$ timedatectl
      Local time: Wed 2018-02-14 13:21:30 GMT
  Universal time: Wed 2018-02-14 13:21:30 UTC
        RTC time: Wed 2018-02-14 13:21:30
       Time zone: Europe/London (GMT, +0000)

Context: What we know

  • Time and date values are not stored with timezone information in MySQL (there is no clear description of this in The DATE, DATETIME, and TIMESTAMP Types, except "MySQL converts TIMESTAMP values from the current time zone to UTC for storage, and back from UTC to the current time zone for retrieval. (This does not occur for other types such as DATETIME.)"; note that "converts TIMESTAMP values to UTC for storage" actually makes sense only as an implementor's note, it's not useful to the user – what is important is that TiMEZONE always returns the correct value in whatever timezone one is in, while DATETIME will always return the same value in whatever timezone one is in)

Context: Current time

It's now around :

$ date +"The current time is %s alias %c"
The current time is 1518617572 alias Wed 14 Feb 2018 14:12:52 GMT

Problem

Let's run:

SELECT NOW(),FROM_UNIXTIME(0),UNIX_TIMESTAMP(),FROM_UNIXTIME(UNIX_TIMESTAMP());

The output (suitably pivoted by hand; there should be an option to do that):

NOW()                            2018-02-14 14:15:13  -- Display "now" in SYSTEM timetone: OK
FROM_UNIXTIME(0)                 1970-01-01 01:00:00  -- Display 1970-01-01 00:00:00 in SYSTEM timetone: NOT OK
UNIX_TIMESTAMP()                 1518617713           -- Correct unixtime
FROM_UNIXTIME(UNIX_TIMESTAMP())  2018-02-14 14:15:13  -- Same as NOW(): OK

In the above everything is fine, except the FROM_UNIXTIME(0) giving 1970-01-01 01:00:00, which is one hour off. If Unix Time 0 were displayed in UTC, it would give 1970-01-01 00:00:00 (or would it?)

This happens if I connect locally or from a remote machine.

To make doubly sure, I checked by coercing to DATETIME. The same happens:

CREATE TEMPORARY TABLE test (N DATETIME, FUX DATETIME, UXT LONG, NN DATETIME);
INSERT INTO test SELECT NOW(),FROM_UNIXTIME(0),UNIX_TIMESTAMP(),FROM_UNIXTIME(UNIX_TIMESTAMP());
SELECT * FROM test;
+---------------------+---------------------+------------+---------------------+
| N                   | FUX                 | UXT        | NN                  |
+---------------------+---------------------+------------+---------------------+
| 2018-02-14 14:19:51 | 1970-01-01 01:00:00 | 1518617991 | 2018-02-14 14:19:51 |
+---------------------+---------------------+------------+---------------------+

Best Answer

This does depend on the session time zone. I'm in the same time zone as yourself:

$ timedatectl
    Local time: Wed 2018-02-14 21:25:44 GMT
Universal time: Wed 2018-02-14 21:25:44 UTC
      RTC time: Wed 2018-02-14 21:25:44
     Time zone: Europe/London (GMT, +0000)

The below results are from MariaDB, but I assume it's the same with MySQL.

SET time_zone = 'SYSTEM';
SELECT FROM_UNIXTIME(0), NOW();
+---------------------+---------------------+
| FROM_UNIXTIME(0)    | NOW()               |
+---------------------+---------------------+
| 1970-01-01 01:00:00 | 2018-02-14 21:30:11 |
+---------------------+---------------------+

And:

SET time_zone = '+1:00';
SELECT FROM_UNIXTIME(0), NOW();
+---------------------+---------------------+
| FROM_UNIXTIME(0)    | NOW()               |
+---------------------+---------------------+
| 1970-01-01 01:00:00 | 2018-02-14 22:30:33 |
+---------------------+---------------------+

However:

SET time_zone = '+0:00';
SELECT FROM_UNIXTIME(0), NOW();
+---------------------+---------------------+
| FROM_UNIXTIME(0)    | NOW()               |
+---------------------+---------------------+
| 1970-01-01 00:00:00 | 2018-02-14 21:34:41 |
+---------------------+---------------------+

I'm surprised that time_zone values SYSTEM and +0:00 give different results for FROM_UNIXTIME(0), given that my time zone is GMT, +0000. This looks like a bug to me, but maybe there's something I'm not understanding.

Related Question