Description:
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The UNIX timestamp gives the number of seconds since January 1 1970 00:00:00 UTC. 
But value of UNIX timestamp can be different, that in really is. More information 
on http://www.najdouvas.cz/unix-timestamp-error/

Test script:
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<?php
date_default_timezone_set('UTC');
echo "Current time and date in UTC timezone: " . date("M d Y H:i:s", time()) . "<br>";
date_default_timezone_set('America/Kentucky/Monticello');
echo "Current time and date in America/Kentucky/Monticello timezone: " . date("M d Y H:i:s", time()) . "<br>";
echo "UNIX timestamp defined by Strtotime for now: " . strtotime("now")  . " <br>";
$start = strtotime("now");
$zonestart = date("M d Y H:i:s", 00);
echo "UNIX timestamp start at: Jan 01 1970 00:00:00 UTC<br>";
echo "when convert this time to America/Kentucky/Monticello, it returns following date for Monticello timezone<br>";
echo "UNIX timestamp start at:   " . $zonestart . " America/Kentucky/Monticello timezone<br>";
echo "To this date we add UNIX timestamp for NOW time. The result may be the same as the current time and date for America/Kentucky/Monticello <br>";
$date = date_create($zonestart);
$date->modify('+ ' . $start . ' sec');
echo "Result is: " . $date->format('M d Y H:i:s e') . "<br>";
date_default_timezone_set('UTC');
$date->setTimezone(new DateTimeZone("UTC")); 
echo "Changing this time to UTC timezone: " . $date->format('M d Y H:i:s e') . "<br><br>";
echo "When final date is not the same as the date at the begining, it means, that the UNIX timestamp don't give up exact number of second for this place or timezone. The UNIX timestamp gives the number of seconds since January 1 1970 00:00:00 UTC. But it's different for different places.<br><br>";
?>