$timestamp=1330581600

e.g.
$beginOfDay = Start of Timestamp's Day
$endOfDay = End of Timestamp's Day

$endOfDay = $timestamp + (60 * 60 * 23);

$beginOfDay = strtotime("midnight", $timestamp);
$endOfDay   = strtotime("tomorrow", $beginOfDay) - 1;

$dtNow = new DateTime();
// Set a non-default timezone if needed
$dtNow->setTimezone(new DateTimeZone('Pacific/Chatham'));
$dtNow->setTimestamp($timestamp);

$beginOfDay = clone $dtNow;

// Go to midnight.  ->modify('midnight') does not do this for some reason
$beginOfDay->modify('today');

$endOfDay = clone $beginOfDay;
$endOfDay->modify('tomorrow');
// adjust from the next day to the end of the day, per original question
$endOfDay->modify('1 second ago');

var_dump(
    array(
        'time ' => $dtNow->format('Y-m-d H:i:s e'),
        'start' => $beginOfDay->format('Y-m-d H:i:s e'),
        'end  ' => $endOfDay->format('Y-m-d H:i:s e'),
    )
);

$beginOfDay = DateTime::createFromFormat('Y-m-d H:i:s', (new DateTime())->setTimestamp($timestamp)->format('Y-m-d 00:00:00'))->getTimestamp();
$endOfDay = DateTime::createFromFormat('Y-m-d H:i:s', (new DateTime())->setTimestamp($timestamp)->format('Y-m-d 23:59:59'))->getTimestamp();

$dateTimeObject = new DateTime();
$dateTimeObject->setTimestamp($timestamp);
$beginOfDayString = $dateTimeObject->format('Y-m-d 00:00:00');
$beginOfDayObject = DateTime::createFromFormat('Y-m-d H:i:s', $beginOfDayString);
$beginOfDay = $beginOfDayObject->getTimestamp();

$endOfDayObject = clone $beginOfDayOject(); // Cloning because add() and sub() modify the object
$endOfDayObject->add(new DateInterval('P1D'))->sub(new DateInterval('PT1S'));
$endOfDay = $endOfDayOject->getTimestamp();

$beginOfDay = DateTime::createFromFormat('Y-m-d H:i:s O', (new DateTime())->setTimezone(new DateTimeZone('America/Los_Angeles'))->setTimestamp($timestamp)->format('Y-m-d 00:00:00 O'))->getTimestamp();

list($y,$m,$d) = explode('-', date('Y-m-d', $ts));
$start = mktime(0,0,0,$m,$d,$y);
$end = mktime(0,0,0,$m,$d+1,$y);

$start_of_day = time() - 86400 + (time() % 86400);
$end_of_day = $start_of_day + 86400;

$stamp = mktime(0, 0, 0);
echo date('m-d-Y H:i:s',$stamp);

<?php
$date = "2015-04-12 09:20:00";

$midnight = strtotime("midnight", strtotime($date));
$now = strtotime($date);

$diff = $now - $midnight;
echo $diff;
?>

<?php
$midnight = strtotime("midnight");
$now = date('U');

$diff = $now - $midnight;
echo $diff;
?>

$start_of_day = floor (time() / 86400) * 86400;
$end_of_day = ceil (time() / 86400) * 86400;

$start_of_day = floor (time() / 86400) * 86400;
$end_of_day = $start_of_day + 86400;

$today = date('Y-m-d 00:00:00');