Date validation with html5 dates

This code validates the following:

• enddate should not be less than or equal to startdate.
• startdate should not be less than or equal to the date today unless document.getElementById("ltype").value == "1".
• The maximum gap between startdate and enddate is one year, more than that should cause an error alert.

Please review my code. I think I covered all test cases, but I am extremely new to JavaScript, so this code might have bugs. And is it fine to compare dates as strings? I did this because as far as I know HTML5 date is a string.

// creates a date object then converts it to a string with yyyy-mm-dd format
function dateFormat(date, addyear) { // addyear is boolean, true means add another year to the date
    var dd = date.getDate();
    var mm = date.getMonth()+1;
    var yyyy = date.getFullYear();
    if (addyear) {
        yyyy++;
    }
    if(dd<10) {
        dd='0'+dd;
    } 
    if(mm<10) {
        mm='0'+mm;
    } 
    return yyyy+'-'+mm+'-'+dd;
}

function date_compare() {
    var today = dateFormat(new Date(),false);
    // from and to are html5 date fields
    var d1=document.getElementById("from").value;
    var d2=document.getElementById("to").value;
    var startdate = dateFormat(new Date(d1),false);
    var enddate = dateFormat(new Date(d2),false);
    var yeardate = dateFormat(new Date(d1),true);
    if (enddate <= startdate || enddate > yeardate) {
        alert("Dates out of range");
        return false; 
    }
    if (document.getElementById("ltype").value != "1" && startdate <= today) {
        alert("Error! date goes backwards!");
        return false;
    }
    return true
}