Getting and Storing a timestamp difference using PHP and MySQL - php

Question

I'm creating a basic game for a final project; the entire site is built and ready to go, but I can't get one functionality working.
The function is that over time the user's pet's stats will lower.
The longer you are away from your pet the lower their stats get until they "die". So far I'm trying to calculate the time difference between the logins by using this code:
$mod = mysqli_query($con,"select * from users where user_name='$user_name'");
while($row = mysqli_fetch_array($mod)){
$last_login = $row['last_login'];
$stored_login = $row['stored_login'];
$time_diff = ($last_login-$stored_login);
echo $time_diff;
$time_mod = mysqli_query($con,"update users set time_mod='$time_diff' where user_name='$user_name'");
}
Both the last_login and stored_login are TIMESTAMP variables, and I've been told I should be able to subtract them easily to get a solution.
However, when I echo the equation all it returns is 0. I've tried using DATEDIFF but MySQL gives me an error saying I can't use it. I only need the number of days that have passed - Is there any way to make this happen?
If it helps, you can access the beta here (either create an account or you can log on with):
http://www.eurogabby.com/MyPetMonster/login.php
username: a
password: a

Answer 1

Maybe you can try. Convert to time the row.
$last_login = strtotime($row['last_login']);
$stored_login = strtotime($row['stored_login']);
$time_diff = ($last_login - $stored_login);
echo $time_diff;

Answer 2

you just need to convert the time using strtotime like this
$time_diff = (strtotime($last_login)-strtotime($stored_login));
echo $time_diff;
if this not works then first convert them to strtotime and store in different variables then subtract them like
$last = strtotime($last_login);
$stored = strtotime($stored_login);
$time_diff = ($last-$stored);
echo $time_diff;
Now you can convert back to time using date function like this
$newtime = date("m/d/Y",$time_diff);
echo $newtime = month/day/year
remember m/d/y will be your choice of date format in which you want to convert.