Description:
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From manual page: http://www.php.net/class.datetime
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When creating a DateTime object from ISO string (ie 2014-11-28T12:39:36.257642Z) the microseconds returned by format('u') differ from microseconds in the timestring for certain strings.

Test script:
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$t = "2014-11-28T12:39:36.257642Z";
$d = new DateTime($t);
echo "Input:  " . $t . "\n";
echo "Microseconds from datetime: ". $d->format('u'); // ends in 1

Expected result:
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$t = "2014-11-28T12:39:36.257641Z";
$d = new DateTime($t);
echo "Input:  " . $t . "\n";
echo "Microseconds from datetime: ". $d->format('u'); // Correct