The easiest way to calculate the time difference in PHP is using date_diff function()
<?php
$start = date_create('1988-08-10');
$end = date_create(); // Current time and date
$diff = date_diff( $start, $end );
echo 'The difference is ';
echo $diff->y . ' years, ';
echo $diff->m . ' months, ';
echo $diff->d . ' days, ';
echo $diff->h . ' hours, ';
echo $diff->i . ' minutes, ';
echo $diff->s . ' seconds';
// Output: The difference is 28 years, 5 months, 19 days, 20 hours, 34 minutes, 36 seconds
echo 'The difference in days : ' . $diff->days;
// Output: The difference in days : 10398