$submitted = "Friday, April 11, 2014 02:55 PM"
$duration = "3 Month"
$execution = "Friday, April 13, 2014 02:55 PM"

$percentage = calculate ($execution+$duration) - $submitted
$percentage = $percentage divided by 100%.

$datetime1 = new DateTime('Friday, April 11, 2014 02:55 PM');
$datetime2 = new DateTime('Sunday, April 13, 2014 02:55 PM');
$interval = $datetime1->diff($datetime2);

echo $interval->format('%d Days');

2 Days

print_r($interval) ;

DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 2
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 2
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)