Choose the preferred offset during a DateTime DST overlap - php

Question

In time-zones observing Daylight Saving Time, the clock typically:
moves forward during transition from winter to summer
is set back during transition from summer to winter
For example, in the Europe/Paris time-zone, the UTC offset changes from +02:00 to +01:00 during the transition from summer to winter, at 3:00 AM on the last Sunday of the month of October.
In other words:
At 3:00 AM (+02:00) on 2014-10-26, clocks are set back to 2:00 AM (+01:00).
Which means that creating a DateTime for 2014-10-26 at 02:30 AM in the Europe/Paris time-zone is ambiguous, as it can represent either:
2014-10-26T02:30+01:00 (timestamp 1414287000)
2014-10-26T02:30+02:00 (timestamp 1414283400)
Java's ZonedDateTime documentation explains this problem very well, and their API offers a way to choose the preferred offset if needed.
In PHP however, it seems that this ambiguity is resolved by choosing arbitrarily the winter time:
$dt = new DateTime('2014-10-26T02:30', new DateTimeZone('Europe/Paris'));
echo $dt->format(DateTime::ISO8601); // 2014-10-26T02:30:00+0100
echo $dt->getTimestamp(); // 1414287000
echo $dt->getOffset(); // 3600
echo $dt->getTimeZone->getName(); // Europe/Paris
(By arbitrarily, I mean that I could not find any documentation about it).
Is there a way to choose the preferred offset when creating a DateTime from a date and time that fall within a DST overlap for the given time-zone?
Or in other words:
How can I create a DateTime object that would exhibit the following characteristics:
echo $dt->format(DateTime::ISO8601); // 2014-10-26T02:30:00+0200
echo $dt->getTimestamp(); // 1414283400
echo $dt->getOffset(); // 7200
echo $dt->getTimeZone->getName(); // Europe/Paris
That is, an object representing this date/time in the Europe/Paris time-zone in summer time?

Answer 1

First, consider that there is a known bug in PHP that will affect you here. Consider:
$dt = new DateTime('2014-10-26T02:30', new DateTimeZone('Europe/Paris'));
echo $dt->format(DateTime::ISO8601) . " (" . $dt->getTimeStamp() . ")\n";
$dt->setTimeStamp($dt->getTimeStamp() - 3600);
echo $dt->format(DateTime::ISO8601) . " (" . $dt->getTimeStamp() . ")\n";
Output:
2014-10-26T02:30:00+0100 (1414287000)
2014-10-26T02:30:00+0100 (1414287000)
Even though you adjusted the timestamp back an hour to reflect summer time, PHP erroneously advanced it to the winter time position.
You can work around this for display purposes by using UTC as an intermediary.
$tz = new DateTimeZone('Europe/Paris');
$dt = new DateTime('2014-10-26T02:30', $tz);
echo $dt->format(DateTime::ISO8601) . " (" . $dt->getTimeStamp() . ")\n";
$ts = $dt->getTimeStamp() - 3600;
$dt = new DateTime("#$ts", new DateTimeZone('UTC'));
$dt->setTimeZone($tz);
echo $dt->format(DateTime::ISO8601) . " (" . $dt->getTimeStamp() . ")\n";
Output:
2014-10-26T02:30:00+0100 (1414287000)
2014-10-26T02:30:00+0200 (1414287000)
Note that even though the wrong timestamp is returned (it should be 1414283400), it does retain the desired summer-time offset of +0200.
Now, lets tackle the problem of knowing when to apply this. We'll examine the transitions and use that to decide whether or not to subtract an hour.
// set up the original input values
$tz = new DateTimeZone('Europe/Paris');
$dt = new DateTime('2014-10-26T02:30', $tz);
echo $dt->format(DateTime::ISO8601) . "\n";
// check for a transition +/- an hour from the current time stamp
$ts = $dt->getTimestamp();
$transitions = $tz->getTransitions($ts - 3600, $ts + 3600);
if (count($transitions) > 1) {
// see if we are moving backwards, creating the ambiguity
$shift = $transitions[1]['offset'] - $transitions[0]['offset'];
if ($shift < 0)
{
// apply the difference in offsets to move back to summer time
$ts = $ts + $shift;
$dt = new DateTime("#$ts", new DateTimeZone('UTC'));
$dt->setTimeZone($tz);
}
}
echo $dt->format(DateTime::ISO8601) . "\n";
Output:
2014-10-26T02:30:00+0100
2014-10-26T02:30:00+0200
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