JavaScript Calendar Pull Down Menu

<script type="text/javascript">
function DateSelector(selYear, selMonth, selDay)
{
this.selYear = selYear;
this.selMonth = selMonth;
this.selDay = selDay;
this.selYear.Group = this;
this.selMonth.Group = this;
// add onchange event to year,month
if(window.document.all != null) // IE
{
this.selYear.attachEvent("onchange", DateSelector.Onchange);
this.selMonth.attachEvent("onchange", DateSelector.Onchange);
}
else // Firefox
{
this.selYear.addEventListener("change", DateSelector.Onchange, false);
this.selMonth.addEventListener("change", DateSelector.Onchange, false);
}

if(arguments.length == 4) // if no of argument is 4, the last one is Date format
this.InitSelector(arguments[3].getFullYear(), arguments[3].getMonth() + 1, arguments[3].getDate());
else if(arguments.length == 6) // if no of argument is 4, the last 3 should be year, month, date
this.InitSelector(arguments[3], arguments[4], arguments[5]);
else // take the default date
{
var dt = new Date();
this.InitSelector(dt.getFullYear(), dt.getMonth() + 1, dt.getDate());
}
}

// add minimum year
DateSelector.prototype.MinYear = 1900;

// add maximum year
DateSelector.prototype.MaxYear = (new Date()).getFullYear()+1;

// initialize year
DateSelector.prototype.InitYearSelect = function()
{

for(var i = this.MaxYear; i >= this.MinYear; i--)
{
//create option
var op = window.document.createElement("OPTION");

// set option value
op.value = i;

// set option content
op.innerHTML = i;


this.selYear.appendChild(op);
}
}

// initialize month
DateSelector.prototype.InitMonthSelect = function()
{
for(var i = 1; i < 13; i++)
{
var op = window.document.createElement("OPTION");
op.value = i;
op.innerHTML = i;
this.selMonth.appendChild(op);
}
}

// get days in a month
DateSelector.DaysInMonth = function(year, month)
{
var date = new Date(year, month, 0);
return date.getDate();
}

// initialize date
DateSelector.prototype.InitDaySelect = function()
{
var year = parseInt(this.selYear.value);
var month = parseInt(this.selMonth.value);
var daysInMonth = DateSelector.DaysInMonth(year, month);
this.selDay.options.length = 0;
for(var i = 1; i <= daysInMonth ; i++)
{
var op = window.document.createElement("OPTION");
op.value = i;
op.innerHTML = i;
this.selDay.appendChild(op);
}
}

DateSelector.Onchange = function(e)
{
var selector = window.document.all != null ? e.srcElement : e.target;
selector.Group.InitDaySelect();
}

DateSelector.prototype.InitSelector = function(year, month, day)
{
this.selYear.options.length = 0;
this.selMonth.options.length = 0;

this.InitYearSelect();
this.InitMonthSelect();
this.selYear.selectedIndex = this.MaxYear - year;
this.selMonth.selectedIndex = month - 1;

this.InitDaySelect();

this.selDay.selectedIndex = day - 1;
}
</script>