difference between two given dates without using PHP date functions or objects

i need to calculate difference between two given dates without using PHP date functions or objects here my first code but it dosn't work weel! Any idea/tips?

Thanks

protected function intiDatesFromSmallerToBigger()
    {
        $start = new DateSet($this->startDate);
        $end = new DateSet($this->endDate);

        if ($start->integer > $end->integer)
        {
            $this->fromDate = $end;
            $this->toDate = $start;
            $this->invert = true;
            return;
        }

        $this->fromDate = $start;
        $this->toDate = $end;
    }

    protected function isLeapYear($year)
    {
        return ((($year % 4) == 0) && ((($year % 100) != 0) || (($year %400) == 0)));
    }

    protected function calcDiffDays()
    {
        $increment = 0;
        if ($this->fromDate->day > $this->toDate->day)
        {
            $increment = $this->monthDay[(int)$this->fromDate->month - 1];
        }

        if ($increment == 28 && $this->isLeapYear($this->fromDate->year)){
            $increment = 29;
        }

        $this->days = $this->toDate->day;

        if($increment > 0) {
            $this->days += $increment;
            $this->months = 1;
        }

        if($this->fromDate->month < $this->toDate->month && $this->fromDate->day > $this->toDate->day) {
            $this->days--;
        }

        $this->days -= $this->fromDate->day;
    }

    private function calcDiffMonths()
    {
        $months = $this->toDate->month - ($this->fromDate->month + $this->months);
        if($this->fromDate->month + $this->months > $this->toDate->month) {
            $months += 12;
            $this->years = 1;
        }
        $this->months = $months;
    }

    protected function calcDiffYears()
    {
        $years = $this->toDate->year - ($this->fromDate->year + $this->years);
        $this->years = $years;
    }

    protected function calcDiffTotalDays()
    {
        $total = $this->days;

        $days=0;
        foreach(range($this->fromDate->year,$this->toDate->year) as $year) {
            foreach($this->monthDay as $index=>$monthDays) {
                $month = $index +1;
                if($year == $this->fromDate->year and $month < $this->fromDate->month) {
                    continue;
                }
                if($monthDays == 28 and $this->isLeapYear($year)) {
                    $days++;
                }
                $days += $monthDays;

                if($year == $this->toDate->year and $month > $this->toDate->month) {
                    break;
                }

            }
        }

        //rimuovi i giorni dell'primo mese
        $days -= $this->fromDate->day;
        //rimuovi i giorni dell'ultimo mese
        $days -= ($this->monthDay[$this->toDate->month - 1] - $this->toDate->day);

        $this->totalDays = $days;
    }