Why does DateTimeZone's getOffset() need a parameter?

Question

What's the point of PHP's DateTimeZone::getOffset's (timezone_offset_get) datetime parameter?

<?php

$tz = new DateTimeZone("Asia/Tokyo");

var_dump($tz->getOffset(new DateTime())); // 32400
var_dump($tz->getOffset(new DateTime('now', new DateTimeZone('Asia/Taipei')))); // 32400
var_dump($tz->getOffset(new DateTime('now', new DateTimeZone('Europe/Budapest')))); // 32400
var_dump($tz->getOffset(new DateTime('2000-01-01', new DateTimeZone('America/New_York')))); // 32400

Am I missing something, or the parameter doesn't affect the output of the function at all?

The documentation mentions the function computes the offset from/for the parameter. What does this mean?

AndreKR · Accepted Answer · 2019-02-09 20:41:26Z

3

Because the offset depends on the date.

For example, in summer Europe/Budapest has an offset of 2 hours (because of daylight saving time), in winter of 1 hour:

$tz = new DateTimeZone("Europe/Budapest");

var_dump($tz->getOffset(new DateTime('2018-06-06', $tz))); // 7200
var_dump($tz->getOffset(new DateTime('2018-01-06', $tz))); // 3600

Daylight saving time is not the only source of change, see for example Moscow time:

$tz = new DateTimeZone("Europe/Moscow");

var_dump($tz->getOffset(new DateTime('2010-01-01', $tz))); // 10800
var_dump($tz->getOffset(new DateTime('2012-01-01', $tz))); // 14400
var_dump($tz->getOffset(new DateTime('2015-01-01', $tz))); // 10800

edited Feb 9, 2019 at 20:41

answered Feb 9, 2019 at 20:32

AndreKR

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