PHP adding datetime with time and getting time difference

Question

I have a mysql datetime object, let's call it "purchase date", like so 2014-04-15 14:47:39. I also have a mysql time object, let's call it "shelf life", like so 168:00:00, meaning that product has a shelf life of 168 hours, or seven days, after its purchase date. I would like to know how many days there are until this product expires, with the following formula (or something similar)

daysRemainingBeforeExpiring = [purchase_date (datetime) + shelf_life (time)] - now(datetime)

Please correct me if my formula is wrong, but I believe this should return the number of days before this item expires (or a negative number if it's expired). How can I perform this math operation on the datetime and time objects in PHP?

Answer 1

$purchase_date = new DateTime('2014-04-15 14:47:39');
list($hours, $minutes, $seconds) = explode(':', '168:00:00');
$shelf_life    = new DateInterval("PT{$hours}H{$minutes}M{$seconds}S");
$expires_date  = $purchase_date->add($shelf_life); 
$now           = new DateTime();
$diff          = $now->diff($expires_date);
echo $diff->format('%r%d Days');

Demo