Description:
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When using set time after modifying the date by using sub, the sub is 
re-applied as well.

Reproduce code:
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date_default_timezone_set('Europe/London');

$datetime = new DateTime('2010-02-19 14:52:10');

$datetime->sub(new DateInterval("P2D"));
// Date should now be 2010-02-17
echo $datetime->format("Y-m-d H:i:s") . "<br />";

$datetime->setTime(15,0,0);
// Date and time should now be 2010-02-17 15:00:00
// BUT it is 2010-02-15 15:00:00
echo $datetime->format("Y-m-d H:i:s");

Expected result:
----------------
Only the time should have be changed

Actual result:
--------------
Day and date are changed.