Here comes the DateTime
:
$start_date = '2013-03-06';
$date = DateTime::createFromFormat('Y-m-d',$start_date);
$date->modify('+1 month');
echo $date->format('Y-m-d');//2013-04-06
$date->modify('+4 year');
echo $date->format('Y-m-d');//2017-04-06
$date->modify('+6 day');
echo $date->format('Y-m-d');//2017-04-12
$date->modify('+24 hours');
echo $date->format('Y-m-d');//2017-04-13
$date->modify('-7 years');
echo $date->format('Y-m-d'); //2010-04-13
$date->modify('-18 months');
echo $date->format('Y-m-d'); //2008-10-13
So on and so forth.