Php date('H:i:s')?

как правильно конец дня? 24.00.00 или 00.00.00

так правильно тогда?

switch ($acti)
{
    case 1:
        if ($todaysmen > '00.00.00' and $todaysmen < '08.00.00'){
            $choise = " (\"F_TagName_ID\" = $val1 or \"F_TagName_ID\" = $val2) and \"F_Date\" >(timestamp\'$dt 20:00:00\' - \"interval\" \'1 day\')   and \"F_Date\" <= timestamp\'$dt 8:00:00\' ";
        }
        else 
        {
           $choise = " (\"F_TagName_ID\" = $val1 or \"F_TagName_ID\" = $val2) and \"F_Date\" >(timestamp\'$dt 20:00:00\')   and \"F_Date\" <= (timestamp\'$dt 8:00:00\' + - \"interval\" \'1 day\') ";   
        }
        break;
    case 2 :
        $choise = " (\"F_TagName_ID\" = $val1 or \"F_TagName_ID\" = $val2) and \"F_Date\" >timestamp\'$dt 8:00:00\'    and \"F_Date\" <= timestamp\'$dt 20:00:00\'";
        //$acti  = null;
        break;
    case 3 :
        $choise = " (\"F_TagName_ID\" = $val1 or \"F_TagName_ID\" = $val2) and \"F_Date\" >(timestamp\'$dt 20:00:00\' - \"interval\" \'1 day\')   and \"F_Date\" <= timestamp\'$dt 20:00:00\'";
        //$acti  = null;
        break;
}

$acti - 1 смена с 20.00 до 8.00
2 смена с 8.00 до 20.00
3 - выбран день с 20.00 до 20.00