PHP time manipulation

file.php

<?php

// counter 2018-09-23 10:01:27 to days old
function days_old($time)
{
    $date = strtotime($time);              # Converted to a PHP date (a second count)
    $diff = $date - time();                # time returns current time in seconds
    $days = floor($diff / (60 * 60 * 24)); # seconds/minute*minutes/hour*hours/day)
    return abs($days); // number of days old
}

// 2018-09-23 10:01:27 add 5 days to 2018-09-28 10:01:27
function add_days_old($current_time, $days_add)
{
    $timestamp = strtotime($current_time);                # Converted to a PHP date (a second count)
    $timestamp = $timestamp + $days_add * (60 * 60 * 24); # add more time (a second count)
    return date('Y-m-d H:i:s', $timestamp); // example 2018-09-23 10:01:27
}