Can someone tell me how the DateTimeZone::getOffset PHP function works?

Question

I got this line of code:

echo (new DateTimeZone('UTC'))->getOffset(new DateTime('now', new DateTimeZone('America/New_York')));

It returns zero. But I expected it to return the difference between UTC and America/New_York time zones.

I then provided the same time zone twice:

echo (new DateTimeZone('America/New_York'))->getOffset(new DateTime('now', new DateTimeZone('America/New_York')));

I expected it to return zero because the time zones are the same. But it now returns -18000 (That's the offset between America/New_York and UTC).

In the documentation it says that getOffset() always returns the offset to GMT (=UTC). But then why is getOffset not static? And if it is always the offset relative to GMT why does the time zone in the first constructor play a role?

I know there's another getOffset method in the DateTime class which is easier to use. But i want to understand how the getOffset method in the DateTimeZone class works.

Answer 1

The offset of a timezone is how many hours it differs from UTC. The offset of a timezone may change throughout the year depending on when DST goes into and out of effect. So you cannot get the offset of a timezone just like that, since it's not a constant value. You can only interrogate a timezone what its offset from UTC is at a given time. That's what you pass the DateTime object to getOffset for: to ask the timezone what its offset value is for the given time. E.g.:

echo (new DateTimeZone('America/New_York'))->getOffset(new DateTime), PHP_EOL;
echo (new DateTimeZone('America/New_York'))->getOffset(new DateTime('+6 months'));

-18000
-14400

If you want to get the offset difference between two arbitrary timezones, you get their respective UTC offsets and subtract them.