Description:
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Adding or subtracting milliseconds from a DateTime object does not give the correct result.

Test script:
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<?php

$millisInThePast = 50000;

$currentDate = new DateTime("2019-10-14T15:08:23.123+02:00");
echo 'Current: ' . $currentDate->format(DATE_RFC3339_EXTENDED) . "\n";
echo 'Past:    ' . $currentDate->modify("-{$millisInThePast} msec")->format(DATE_RFC3339_EXTENDED);

Expected result:
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Current: 2019-10-14T15:08:23.123+02:00
Past:    2019-10-14T15:07:33.123+02:00

Actual result:
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Current: 2019-10-14T15:08:23.123+02:00
Past:    2019-10-14T15:08:21.-47877+02:00