Calendar PHP - error at first day

Javascript - functions enable swith months

        function goLastMonth(month, year)
        {
            if (month == 1) 
            {
                --year;
                month = 13;
            }
            --month

            var monthstring= ""+month+"";
            var monthlength = monthstring.length;

            if(monthlength <=1)
            {
                monthstring = "0" + monthstring;
            }

        document.location.href ="<?php $_SERVER['PHP_SELF'];?>?month="+monthstring+"&year="+year;
        }


        function goNextMonth(month, year)
        {
            if(month == 12) 
            {
                ++year;
                month = 0;
            }

            ++month
            var monthstring= ""+month+"";
            var monthlength = monthstring.length;

            if(monthlength <=1)
            {
                monthstring = "0" + monthstring;
            }
            document.location.href ="<?php $_SERVER['PHP_SELF'];?>?month="+monthstring+"&year="+year;
        }

        </script>

PHP - creating calendar

            if (isset($_GET['day']))
            {
                $day = $_GET['day'];
            } 
            else 
            {
                $day = date("j");
            }

            if(isset($_GET['month']))
            {
                $month = $_GET['month'];
            } 
            else 
            {
                $month = date("n");
            }

            if(isset($_GET['year']))
            {
                $year = $_GET['year'];
            }
            else
            {
                $year = date("Y");
            }

            $currentTimeStamp = strtotime( "$day-$month-$year");
            $monthName = date("F", $currentTimeStamp);
            $numDays = date("t", $currentTimeStamp);
            $counter = 0;

        ?>


        <table border='1'>
            <tr>
                <td><input style='width:50px;' type='button' value='<'name='previousbutton' onclick ="goLastMonth(<?php echo $month.",".$year?>)"></td>
                <td colspan='5'><?php echo $monthName.", ".$year; ?></td>
                <td><input style='width:50px;' type='button' value='>'name='nextbutton' onclick ="goNextMonth(<?php echo $month.",".$year?>)"></td>
            </tr>

            <tr>
                <td width='50px'>Poniedziałek</td>
                <td width='50px'>Wtorek</td>
                <td width='50px'>Środa</td>
                <td width='50px'>Czwartek</td>
                <td width='50px'>Piątek</td>
                <td width='50px'>Sobota</td>
                <td width='50px'>Niedziela</td>
            </tr>

            <?php
                echo "<tr>";

                for($i = 1; $i < $numDays+1; $i++, $counter++)
                {
                    $timeStamp = strtotime("$year-$month-$i");

                    if($i == 1) 
                    {
                        $firstDay = date("w", $timeStamp);

                        for($j = 0; $j < $firstDay-1; $j++, $counter++) 
                        {
                            echo "<td>&nbsp;</td>";
                        }
                    }

                    if($counter % 7 == 0) 
                    {
                        echo"</tr><tr>";
                    }

                    $monthstring = $month;
                    $monthlength = strlen($monthstring);
                    $daystring = $i;
                    $daylength = strlen($daystring);



                    echo "<td align='center' ";

                    echo "><a href='".$_SERVER['PHP_SELF']."?month=".$monthstring."&day=".$daystring."&year=".$year."'>".$i."</a></td>";

                }
                echo "</tr>";
            ?>
        </table>

Calender works good, but when the first day in month is sunday, then it starts count days from monday (For example may 2016). How to fix it?