PHP和DST转换问题

Question

I'm having trouble understanding how my code adapts to DST, as it's incorrect with the recent update. I'm storing a date time in the database based on UTC and then converting it back to the local timezone for display. If PHP is taking DST into account, something else is wrong because all of my stored dates are 1 hour off.

$stored_date = '2016-11-16 12:04:01'; // in UTC

$dateTime = new DateTime($stored_date, new DateTimeZone('UTC'));
$dateTimeZone = new DateTimeZone('America/New_York');
$dateTime->setTimezone($dateTimeZone);

print_r($dateTime);

Last week, before DST ended, this would have printed out 2016-11-16 08:04:01. This week, now that DST has ended, it prints out 2016-11-16 07:04:01. Why the hour difference if PHP is properly handing the DST shift?

It shouldn't matter the server settings (I don't think) because I'm explicitly doing the conversion within PHP, right?

I'm ready to start doing a check with PHP to see if DST is in effect and offsetting the conversion by 1 hour because I can't figure out why that hour isn't being automatically compensated for within the DateTime class.

Answer 1

New York city switches between these time zones:

• Winter: EST (Eastern Standard Time) = UTC -5
• Summer: EDT (Eastern Daylight Time) = UTC -4

According to timeanddate.com the switch will happen on 6th of November. Thus the result is correct: 12 - 5 = 7

In other words, PHP is perfectly aware of DST, as we can see in the following code:

$dateTime = new DateTime('2016-11-05 12:04:01', new DateTimeZone('UTC'));
$dateTime->setTimezone(new DateTimeZone('America/New_York'));
echo $dateTime->format('r') . PHP_EOL;

$dateTime = new DateTime('2016-11-06 12:04:01', new DateTimeZone('UTC'));
$dateTime->setTimezone(new DateTimeZone('America/New_York'));
echo $dateTime->format('r') . PHP_EOL;

Sat, 05 Nov 2016 08:04:01 -0400
Sun, 06 Nov 2016 07:04:01 -0500

You can inspect the exact information available in your system's time database:

$timeZone = new DateTimeZone('America/New_York');
print_r($timeZone->getTransitions(mktime(0, 0, 0, 1, 1, 2016), mktime(0, 0, 0, 12, 31, 2016)));

Array
(
    [0] => Array
        (
            [ts] => 1451602800
            [time] => 2015-12-31T23:00:00+0000
            [offset] => -18000
            [isdst] => 
            [abbr] => EST
        )

    [1] => Array
        (
            [ts] => 1457852400
            [time] => 2016-03-13T07:00:00+0000
            [offset] => -14400
            [isdst] => 1
            [abbr] => EDT
        )

    [2] => Array
        (
            [ts] => 1478412000
            [time] => 2016-11-06T06:00:00+0000
            [offset] => -18000
            [isdst] => 
            [abbr] => EST
        )

)