why the day not equal to 24 hours timezone set to los angeles?

Background: our server PHP set timezone to America/Los_Angeles

At work, I found this piece of code

$a = 1457856000;
$b = $a + 24*3600-1;  

var_dump(date('Y-m-d H:i:s', $a));
var_dump(date('Y-m-d H:i:s', $b));

which output:

string(19) "2016-03-13 00:00:00" 
string(19) "2016-03-14 00:59:59"

quite strange, but when I code as follow:

    date_default_timezone_set('Etc/GMT+8');

    $a = 1457856000;
    $b = $a + 24*3600-1;  

    var_dump(date('Y-m-d H:i:s', $a));
    var_dump(date('Y-m-d H:i:s', $b));

it output:

string(19) "2016-03-13 00:00:00"
string(19) "2016-03-13 23:59:59"

Could anyone explain why this happened?