Description:
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When you run the test script, the second outpot is parsed false. It returns the date minus one day. Maybe this error occures with other strings also.

Test script:
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<?php

$dt = new DateTime('-1999-06-22 00:00:00');
echo "\nThis should be -1999-06-22 00:00:00 -> " . $dt->format('Y-m-d H:i:s');

$dt = new DateTime('-1916-06-22 00:00:00');
echo "\nThis should be -1916-06-22 00:00:00 -> " . $dt->format('Y-m-d H:i:s');

Expected result:
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This should be -1999-06-22 00:00:00 -> -1999-06-22 00:00:00
This should be -1916-06-22 00:00:00 -> -1916-06-22 00:00:00

Actual result:
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This should be -1999-06-22 00:00:00 -> -1999-06-22 00:00:00
This should be -1916-06-22 00:00:00 -> -1916-06-21 00:00:00