This function allows the addition of day(s),month(s),year(s) to the original date while still preserving the Hours, minutes and seconds
You can also modify to add to hours, miuntes and even seconds.
 
<?php
function add_date($givendate,$day=0,$mth=0,$yr=0) {
      $cd = strtotime($givendate);
      $newdate = date('Y-m-d h:i:s', mktime(date('h',$cd),
    date('i',$cd), date('s',$cd), date('m',$cd)+$mth,
    date('d',$cd)+$day, date('Y',$cd)+$yr));
      return $newdate;
              }

?>

主要用strtotime函数，可以看看手册。

date('Y-m-d H:i:s', strtotime(date('Y-m-d')." 10:00:00 +1 day"))