将日期转换为其他时区时，您需要存储时间吗？

Question

When using PHP's DateTime to convert a stored date to a different timezone later, do you need to store the time information (e.g., 2012-12-31 00:00:00 instead of just 2012-12-31)? It appears that if I just store the date, the timezone conversion is inaccurate so I'm not sure if time information is really needed or I'm just missing something?

Answer 1

Timezone conversions work by hour offsets - with out the time stored (atleast hours), you can't accurately calculate the difference.

An example to consider - say you want to group data by day, the dates are stored in UTC but your client is in PST.

A row was created on 2012-12-31 08:00:00 in UTC, but you stored 2012-12-31.

$date = new DateTime('2012-12-31', new DateTimeZone('UTC') );
echo $date->setTimezone( new DateTimeZone('America/Los_Angeles') )->format('Y-m-d');
//2012-12-30

$datetime = new DateTime('2012-12-31 08:00:00', new DateTimeZone('UTC') );
echo $datetime->setTimezone( new DateTimeZone('America/Los_Angeles') )->format('Y-m-d');
//2012-12-31

The date should reflect 2012-12-31 in PST, but with out the hour offset it would be converted incorrectly.

Also, as Samuel Cook answered, unless you explicitly store the datetime in your database under a single timezone, you should store the timezone as well, so you know what your converting from.