It seems that it does not matter what timezone is on the server as long as you have the time set right for the current timezone, know the timezone of the datetime columns that you store, and are aware of the issues with daylight savings time.
On the other hand if you have control of the timezones of the servers you work with then you can have everything set to UTC internally and never worry about timezones and DST.
Here are some notes I collected of how to work with timezones as a form of cheatsheet for myself and others which might influence what timezone the person will choose for his/her server and how he/she will store date and time.
MySQL Timezone Cheatsheet
Notes:
Changing the timezone will not change the stored datetime or
timestamp, but it will select a different datetime from
timestamp columns
Warning! UTC has leap seconds, these look like '2012-06-30 23:59:60' and can
be added randomly, with 6 months prior notice, due to the slowing of
the earths rotation
GMT confuses seconds, which is why UTC was invented.
Warning! different regional timezones might produce the same datetime value due
to daylight savings time
The timestamp column only supports dates 1970-01-01 00:00:01 to 2038-01-19 03:14:07 UTC, due to a limitation.
Internally a MySQL timestamp column is stored as UTC but
when selecting a date MySQL will automatically convert it to the
current session timezone.
When storing a date in a timestamp, MySQL will assume that the date
is in the current session timezone and convert it to UTC for
storage.
MySQL can store partial dates in datetime columns, these look like
"2013-00-00 04:00:00"
MySQL stores "0000-00-00 00:00:00" if you set a datetime column as
NULL, unless you specifically set the column to allow null when you
create it.
Read this
To select a timestamp column in UTC format
no matter what timezone the current MySQL session is in:
SELECT
CONVERT_TZ(`timestamp_field`, @@session.time_zone, '+00:00') AS `utc_datetime`
FROM `table_name`
You can also set the sever or global or current session timezone to UTC and then select the timestamp like so:
SELECT `timestamp_field` FROM `table_name`
To select the current datetime in UTC:
SELECT UTC_TIMESTAMP();
SELECT UTC_TIMESTAMP;
SELECT CONVERT_TZ(NOW(), @@session.time_zone, '+00:00');
Example result: 2015-03-24 17:02:41
To select the current datetime in the session timezone
SELECT NOW();
SELECT CURRENT_TIMESTAMP;
SELECT CURRENT_TIMESTAMP();
To select the timezone that was set when the server launched
SELECT @@system_time_zone;
Returns "MSK" or "+04:00" for Moscow time for example, there is (or was) a MySQL bug where if set to a numerical offset it would not adjust the Daylight savings time
To get the current timezone
SELECT TIMEDIFF(NOW(), UTC_TIMESTAMP);
It will return 02:00:00 if your timezone is +2:00.
To get the current UNIX timestamp (in seconds):
SELECT UNIX_TIMESTAMP(NOW());
SELECT UNIX_TIMESTAMP();
To get the timestamp column as a UNIX timestamp
SELECT UNIX_TIMESTAMP(`timestamp`) FROM `table_name`
To get a UTC datetime column as a UNIX timestamp
SELECT UNIX_TIMESTAMP(CONVERT_TZ(`utc_datetime`, '+00:00', @@session.time_zone)) FROM `table_name`
Get a current timezone datetime from a positive UNIX timestamp integer
SELECT FROM_UNIXTIME(`unix_timestamp_int`) FROM `table_name`
Get a UTC datetime from a UNIX timestamp
SELECT CONVERT_TZ(FROM_UNIXTIME(`unix_timestamp_int`), @@session.time_zone, '+00:00')
FROM `table_name`
Get a current timezone datetime from a negative UNIX timestamp integer
SELECT DATE_ADD('1970-01-01 00:00:00',INTERVAL -957632400 SECOND)
There are 3 places where the timezone might be set in MySQL:
Note: A timezone can be set in 2 formats:
- an offset from UTC: '+00:00', '+10:00' or '-6:00'
- as a named time zone: 'Europe/Helsinki', 'US/Eastern', or 'MET'
Named time zones can be used only if the time zone information tables
in the mysql database have been created and populated.
in the file "my.cnf"
default_time_zone='+00:00'
or
timezone='UTC'
@@global.time_zone variable
To see what value they are set to
SELECT @@global.time_zone;
To set a value for it use either one:
SET GLOBAL time_zone = '+8:00';
SET GLOBAL time_zone = 'Europe/Helsinki';
SET @@global.time_zone='+00:00';
@@session.time_zone variable
SELECT @@session.time_zone;
To set it use either one:
SET time_zone = 'Europe/Helsinki';
SET time_zone = "+00:00";
SET @@session.time_zone = "+00:00";
both "@@global.time_zone variable" and "@@session.time_zone variable" might return "SYSTEM" which means that they use the timezone set in "my.cnf".
For timezone names to work (even for default-time-zone) you must setup your timezone information tables need to be populated: http://dev.mysql.com/doc/refman/5.1/en/time-zone-support.html
Note: you can not do this as it will return NULL:
SELECT
CONVERT_TZ(`timestamp_field`, TIMEDIFF(NOW(), UTC_TIMESTAMP), '+00:00') AS `utc_datetime`
FROM `table_name`
Setup mysql timezone tables
For CONVERT_TZ to work, you need the timezone tables to be populated
SELECT * FROM mysql.`time_zone` ;
SELECT * FROM mysql.`time_zone_leap_second` ;
SELECT * FROM mysql.`time_zone_name` ;
SELECT * FROM mysql.`time_zone_transition` ;
SELECT * FROM mysql.`time_zone_transition_type` ;
If they are empty, then fill them up by running this command
mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root -p mysql
if this command gives you the error "data too long for column 'abbreviation' at row 1", then it might be caused by a NULL character being appended at the end of the timezone abbreviation
the fix being to run this
mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root -p mysql
(if the above gives error "data too long for column 'abbreviation' at row 1")
mysql_tzinfo_to_sql /usr/share/zoneinfo > /tmp/zut.sql
echo "SET SESSION SQL_MODE = '';" > /tmp/mysql_tzinfo_to.sql
cat /tmp/zut.sql >> /tmp/mysql_tzinfo_to.sql
mysql --defaults-file=/etc/mysql/my.cnf --user=verifiedscratch -p mysql < /tmp/mysql_tzinfo_to.sql
(make sure your servers dst rules are up to date zdump -v Europe/Moscow | grep 2011 https://chrisjean.com/updating-daylight-saving-time-on-linux/)
See the full DST (Daylight Saving Time) transition history for every timezone
SELECT
tzn.Name AS tz_name,
tztt.Abbreviation AS tz_abbr,
tztt.Is_DST AS is_dst,
tztt.`Offset` AS `offset`,
DATE_ADD('1970-01-01 00:00:00',INTERVAL tzt.Transition_time SECOND) AS transition_date
FROM mysql.`time_zone_transition` tzt
INNER JOIN mysql.`time_zone_transition_type` tztt USING(Time_zone_id, Transition_type_id)
INNER JOIN mysql.`time_zone_name` tzn USING(Time_zone_id)
-- WHERE tzn.Name LIKE 'Europe/Moscow' -- Moscow has weird DST changes
ORDER BY tzt.Transition_time ASC
CONVERT_TZ also applies any necessary DST changes based on the rules in the above tables and the date that you use.
Note:
According to the docs, the value you set for time_zone does not change, if you set it as "+01:00" for example, then the time_zone will be set as an offset from UTC, which does not follow DST, so it will stay the same all year round.
Only the named timezones will change time during daylight savings time.
Abbreviations like CET will always be a winter time and CEST will be summer time while +01:00 will always be UTC time + 1 hour and both won't change with DST.
The system timezone will be the timezone of the host machine where mysql is installed (unless mysql fails to determine it)
You can read more about working with DST here
When not to use UTC by the legendary Jon Skeet: https://codeblog.jonskeet.uk/2019/03/27/storing-utc-is-not-a-silver-bullet/
(For example a scheduled event in the future that represents a time, not an instant in time)
related questions:
Sources:
