Description:
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I know that UTC offset is not the better approach to solve timezone issues, even more than region has DST. But in some situations the offset is the only way that you have to manipulate Timezone.

In this situation with you interact with a DateTime object setting TimeZone more the once, it can cause some strange situations, some cases, event timestamp from DateTime appears to be changed.

Test script:
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I got some situations with DateTime and DateTimeZone appears not showing data in the right way.

// example 1 - Timestamp is changing
$datetime = new DateTime('2016-11-09 20:00:00', new DateTimeZone('UTC'));
var_dump($datetime->getTimestamp());
$datetime->setTimeZone(new DateTimeZone('-03:00'));
$datetime->setTimeZone(new DateTimeZone('-03:00'));
var_dump($datetime->getTimestamp());

// example 2 - Timestamp keeps if you use getTimestamp() before second setTimeZone() calls
$datetime = new DateTime('2016-11-09 20:00:00', new DateTimeZone('UTC'));
var_dump($datetime->getTimestamp());
$datetime->setTimeZone(new DateTimeZone('-03:00'));
$datetime->getTimestamp();
$datetime->setTimeZone(new DateTimeZone('-03:00'));
var_dump($datetime->getTimestamp());