Date incrementor in JavaScript

I know, this isn't Python, this is JavaScript, but I need to back up this code snippet.

I have an HTML page with a date input box that I need to increment by one day forward or backward. See example here: http://home.chpc.utah.edu/~u0553130/Brian_Blaylock/ksl_ozone_viewer.php

So, I here are some JavaScript functions that change the date input +/- one day.

function pad(number, length) {

    //I just use this to pad the day or month integer with zeros
    var str = '' + number;

    while (str.length < length) {

        str = '0' + str;

    }

    return str;

}

function next_day(){

    //                    dec   jan   feb   mar   arp   may   jun   jul   aug   set   oct   nov   dec   jan

    var days_per_month = ["31", "31", "28", "31", "30", "31", "30", "31", "31", "30", "31", "30", "31", "31"];

    

    year = parseInt(document.getElementById('dateinput').value.slice(0,4));

    month = parseInt(document.getElementById('dateinput').value.slice(5,7));

    day = parseInt(document.getElementById('dateinput').value.slice(8,10));

        

    if (day < parseInt(days_per_month[month])){

       day = day+1; 

       day = pad(day,2);

       month = pad(month,2);

        }

    else{

        day = '01';

         if (month==12){

            month = '01';

            year = year +1

            year = pad(year,4)

         }

         else{

            month = month + 1;

            month = pad(month,2);     

         }

        

    }

    

    

    document.getElementById('dateinput').value = year+'-'+month+'-'+day

}

function previous_day(){

    //                    dec   jan   feb   mar   arp   may   jun   jul   aug   set   oct   nov   dec

    var days_per_month = ["31", "31", "28", "31", "30", "31", "30", "31", "31", "30", "31", "30", "31"];

    

    year = parseInt(document.getElementById('dateinput').value.slice(0,4));

    month = parseInt(document.getElementById('dateinput').value.slice(5,7));

    day = parseInt(document.getElementById('dateinput').value.slice(8,10));

    

    if (day == 1){

       day = days_per_month[month-1];

        if (month==1){

            month = '12';

            year = year -1

            year = pad(year,4)

        }

        else{

          month = month-1;

           month = pad(month,2);     

        }

       

    }

    else{

        day = day-1;

        day = pad(day,2);

        month = pad(month,2);

    }

    

    

    document.getElementById('dateinput').value = year+'-'+month+'-'+day;

}

Note: this doesn't allow for a leap year date.