Description:
------------
B - (B - A) == A SHOULD HOLD TRUE
DOES HOLD TRUE USING UNIX TIMESTAMP MATH
DOES NOT HOLD TRUE USING DateTime:sub()

Test script:
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/*
   METHODOLOGY:
      B - (B - A) == A SHOULD HOLD TRUE
      DOES HOLD TRUE USING UNIX TIMESTAMP MATH
      DOES NOT HOLD TRUE USING DateTime:sub()
*/

$DF_MYSQL = 'Y-m-d H:i:s';

$dtB = DateTime::createFromFormat($DF_MYSQL, '2015-04-02 09:44:47');
$dtA = DateTime::createFromFormat($DF_MYSQL, '2014-02-16 02:00:00');

// DateInterval: B-A
$diDiff = $dtA->diff($dtB);

// Unix Timestamp: B-A
$tsDiff = $dtB->getTimestamp() - $dtA->getTimestamp();

// REPORT DIFFERENCES
/*
   NOTE: DIFFERENCE B-A IS
      $y => 1
      $m => 1
      $d => 17
      $h => 7
      $i => 44
      $s => 47
*/
print_r($diDiff);

// Unix Timestamp: CALCULATE A FROM (B - DIFFERENCE) AND REPORT
// NOTE: CORRECT RESULT, SAME AS $dtA: 2014-02-16 02:00:00
$tsShift = $dtB->getTimestamp() - $tsDiff;
$dtOut = new DateTime();
$dtOut->setTimestamp($tsShift);
print_r($dtOut);

// DateInterval: CALCULATE A FROM (B - DIFFERENCE) AND REPORT
// NOTE: INCORRECT RESULT, DIFFERENT FROM $dtA: 2014-02-13 02:00:00
$dtB->sub($diDiff);
print_r($dtB);

Expected result:
----------------
SEE SCRIPT COMMENTS

Actual result:
--------------
SEE SCRIPT COMMENTS