<?php
$date = date("Y-m-d H:i:s");
// current time; viz since I'm in India, it'll set Current Indian Time i.e. 2014-04-28 02:31:58 (exactly like this)
$info['lastboot'] = "2014-04-27 01:55:55";
// I'm assuming this as your last boot time stored in your DB
//Now to compare both the time value, convert them to exact seconds using PHP buit-in function `strtotime()`
//What strtotime() does is convert the string date viz. 2014-04-28 02:31:58 into seconds.
//These seconds are counted from the UNIX EPOCH i.e since 01 Jan 1970 00:00:00
$date_to_sec_1 = strtotime($date);
$date_to_sec_2 = strtotime($info['lastboot']);
if($date_to_sec_1 > $date_to_sec_2)
{
echo "You need to wait for ".($date_to_sec_1-$date_to_sec_2)." seconds before next boot";
}
else
{
// else part
}