This is to help people with calculating time. It's not a perfect example but it gets to the point. Hope it helps somebody :)

calculating.php

<?php
$a = '1371688131';
$b = time();
$diff = ($b - $a);

if ($diff/60 > 30) {
  echo "This is over 30 minutes";
} else {
  echo "This is " . $diff/60 . " minutes";
}

echo "<hr />";

$str = "27.8166666667";
$minusthis = "30";

$negit = ($minusthis - substr($str, 0, 2));

echo "Time until unlocked " . $negit . " minutes";