时区和播出日期

Question

I am trying to convert airing date times of TV episodes. The API I use returns me this exact values:

Datetime: 2015-05-31 21:00
Country: US
Timezone: GMT-5 +DST

Now I'm trying to retrieve the airing datetime in my timezone (Europe/Rome) with the given data. What I'm doing is:

$date = new DateTime('2015-05-31 21:00', new DateTimeZone('GMT-5 +DST'));
$date->setTimezone(new DateTimeZone('Europe/Rome'));
echo $date->format('Y-m-d H:i:s');

Which prints:

2015-06-01 04:00:00

I have some doubts about being the correct method because other sites that give this same information are saying that the above episode, in my country (IT/Italy) has aired on 1st June, at 03:00 (and not 04:00).

So am I doing correctly, and the sites I'm comparing the results wrong?

Answer 1

You get the same value if you use new DateTimeZone('GMT-5'). GMT-5 +DST is not a valid value for a timezone name and probably the constructor of class DateTimeZone uses as much as it can successfully parse from the argument provided to it.

I think you should parse the value of Timezone: "by hand" and adjust the offset by 1 hour if the string ends with +DST.

If you know that the returned timezone is always GMT-5 then you can simply check if the string is GMT-5 +DST and use GMT-4 instead.

Otherwise you can try to parse the received timezone using regular expressions:

// The timezone received from the API
$timezone = 'GMT-5 +DST';

$m = array();
if (preg_match('/^GMT([+-]\d+)( \+DST)?$/', $timezone, $m)) {
    if ($m[2] == ' +DST') {
        // Compute the correct offset using DST
        $tz = sprintf('Etc/GMT%+d', -(intval($m[1])+1));
    } else {
        // No DST; use the correct name of the time zone
        $tz = sprintf('Etc/GMT%+d', -intval($m[1]));
    }
} else {
    // The timezone name has a different format; use it as is
    // You should do better checks here
    $tz = $timestamp;
} 
$date = new DateTime('2015-05-31 21:00', new DateTimeZone($tz));
$date->setTimezone(new DateTimeZone('Europe/Rome'));
echo $date->format('Y-m-d H:i:s');

Update: As @matt-johnson notices, the correct name of the GMT time zones is Etc/GMT followed by the offset.

PHP 5.5 and PHP 5.6 accept and correctly interpret the GMT timezones without the Etc/ prefix. Older versions (5.3, 5.4) throw an exception with message 'DateTimeZone::__construct(): Unknown or bad timezone (GMT-4)'

I updated the code above to use the correct names. There are some things you must notice in the line:

$tz = sprintf('Etc/GMT%+d', -(intval($m[1])+1));

• the + sign on the sprintf() format string forces the + sign to be produced in front of the number (if the number is positive); for negative numbers a - sign is produced with or without it;
• the +1 in the intval($m[1])+1 does the DST correction;
• the sign of the computed value is changed (notice the - in front of it) because of a strange behaviour of the timezone database used by PHP; the behaviour is explained in the documentation:

  Warning:

  Please do not use any of the timezones listed here (besides UTC), they only exist for backward compatible reasons.

  Warning

  If you disregard the above warning, please also note that the IANA timezone database that provides PHP's timezone support uses POSIX style signs, which results in the Etc/GMT+n and Etc/GMT-n time zones being reversed from common usage.

This means GMT-5 is converted by the code above to Etc/GMT+5 and GMT-5 +DST is converted to Etc/GMT+4.