Support user time zones - php

Question

I allow my users to add events to my webapp. Users can originate from all around the world.
My current idea is to determine the user's time zone automatically (using an IP to location API) and insert it into the users table (I will also allow them to change it).
On the events table I will insert the event start date/end date in UTC.
Then, whenever I need to display the event info, I would take the event start date/end date from the table and do the calculation against the user's timezone.
Is that considered as good practice or is there a better way to do that?
Anything I should be aware of when doing this?
Thanks,

Answer 1

Yes, that is indeed a good way. Also keep in mind that if you use the TIMESTAMP type in MySQL, MySQL handles the timezone converting for you. When you insert new dates/times to the database, MySQL converts it from the connection's timezone to UTC (TIMESTAMP is always stored in UTC). When you retrieve a TIMESTAMP field from database, MySQL converts it back to the connection's timezone.
So if you use TIMESTAMP fields in MySQL, all you need to do is tell the user's timezone to MySQL at start of each your page. You do so by:
SET time_zone = 'Europe/Helsinki'
You can also use numeric timezones:
SET time_zone = '+02:00'
Keep in mind that you might need to install the tzinfo to MySQL first, which is trivial though (only for the non-numeric version though). Here's information about how to do it: http://dev.mysql.com/doc/refman/5.0/en/mysql-tzinfo-to-sql.html
In a nutshell, this is the important part:
mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root mysql
Here's an example of how it works:
mysql> CREATE TEMPORARY TABLE test(foo TIMESTAMP);
Query OK, 0 rows affected (0.00 sec)
mysql> SET time_zone = '+00:00';
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO test VALUES ('2011-02-03 16:00:00');
Query OK, 1 row affected (0.00 sec)
mysql> SET time_zone = '+02:00';
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT foo FROM test;
+---------------------+
| foo |
+---------------------+
| 2011-02-03 18:00:00 |
+---------------------+
1 row in set (0.00 sec)
If you don't use TIMESTAMP field, eg. you use DATETIME (which also supports wider range of dates), then you just need to make sure you always insert dates in UTC; I do this by always setting connection's timezone to +00:00. Then you can have a view helper in PHP that converts the datetime to the user's timezone, which is quite trivial to do with PHP's DateTime class and setTimezone function. There's an example in the last link. To use this method, you must make sure PHP is also set to use UTC as its default timezone, which you can do with this:
date_default_timezone_set('UTC');
Whichever method you use, you should always be aware of these facts:
The PHP and MySQL connection's timezones should always be set to the same value so they're consistent with each other
Generally it's a bad idea to mix TIMESTAMP and DATETIME types with each other
If you use TIMESTAMP type, set the timezone to the user's timezone
If you use DATETIME type, set the timezone to UTC and handle timezone convertions in PHP

Answer 2

Few thing I would consider:
Provide the users the means to set the time zone manually.
Assuming your users may change their location, let them specify the time zone for the event they add (or the location if you can get the time zome from that).
This way hopefully they won't miss anything :) Good luck with your webapp!