Description:
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In time zones that make a daylight savings change at midnight, on the day of the daylight savings change, ->modify('-1 second') does not work as expected. America/Santiago, America/Havana, Asia/Tehran, America/Asuncion are a few of the time zones that have this behavior

Test script:
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$timezone = 'America/Santiago';
$dateString = "2020-09-06";
$dt = new \DateTime($dateString);
$dtz = new \DateTimeZone($timezone);
$dt->setTimezone($dtz);

echo 'the start of the day:<br>';
$dt->modify('today');
echo $dt->getTimestamp();

echo '<br><br>the start of the day, minus one second. this is incorrect:<br>';
$dt->modify('-1 second');
echo $dt->getTimestamp();

Expected result:
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1599364799

Actual result:
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1599368399