y el codigo del calendario.js es el siguente:
function positionInfo(object) {
var p_elm = object;
this.getElementLeft = getElementLeft;
function getElementLeft() {
var x = 0;
var elm;
if(typeof(p_elm) == "object"){
elm = p_elm;
} else {
elm = document.getElementById(p_elm);
}
while (elm != null) {
if(elm.style.position == 'relative') {
break;
}
else {
x += elm.offsetLeft;
elm = elm.offsetParent;
}
}
return parseInt(x);
}
this.getElementWidth = getElementWidth;
function getElementWidth(){
var elm;
if(typeof(p_elm) == "object"){
elm = p_elm;
} else {
elm = document.getElementById(p_elm);
}
return parseInt(elm.offsetWidth);
}
this.getElementRight = getElementRight;
function getElementRight(){
return getElementLeft(p_elm) + getElementWidth(p_elm);
}
this.getElementTop = getElementTop;
function getElementTop() {
var y = 0;
var elm;
if(typeof(p_elm) == "object"){
elm = p_elm;
} else {
elm = document.getElementById(p_elm);
}
while (elm != null) {
if(elm.style.position == 'relative') {
break;
}
else {
y+= elm.offsetTop;
elm = elm.offsetParent;
}
}
return parseInt(y);
}
this.getElementHeight = getElementHeight;
function getElementHeight(){
var elm;
if(typeof(p_elm) == "object"){
elm = p_elm;
} else {
elm = document.getElementById(p_elm);
}
return parseInt(elm.offsetHeight);
}
this.getElementBottom = getElementBottom;
function getElementBottom(){
return getElementTop(p_elm) + getElementHeight(p_elm);
}
}
function CalendarControl() {
var calendarId = 'CalendarControl';
var currentYear = 0;
var currentMonth = 0;
var currentDay = 0;
var selectedYear = 0;
var selectedMonth = 0;
var selectedDay = 0;
var months = ['Enero','Febrero','Marzo','Abril','Mayo','Junio',' Julio','Agosto','Septiembre','Octubre','Noviembre' ,'Diciembre'];
var dateField = null;
function getProperty(p_property){
var p_elm = calendarId;
var elm = null;
if(typeof(p_elm) == "object"){
elm = p_elm;
} else {
elm = document.getElementById(p_elm);
}
if (elm != null){
if(elm.style){
elm = elm.style;
if(elm[p_property]){
return elm[p_property];
} else {
return null;
}
} else {
return null;
}
}
}
function setElementProperty(p_property, p_value, p_elmId){
var p_elm = p_elmId;
var elm = null;
if(typeof(p_elm) == "object"){
elm = p_elm;
} else {
elm = document.getElementById(p_elm);
}
if((elm != null) && (elm.style != null)){
elm = elm.style;
elm[ p_property ] = p_value;
}
}
function setProperty(p_property, p_value) {
setElementProperty(p_property, p_value, calendarId);
}
function getDaysInMonth(year, month) {
return [31,((!(year % 4 ) && ( (year % 100 ) || !( year % 400 ) ))?29:28),31,30,31,30,31,31,30,31,30,31][month-1];
}
function getDayOfWeek(year, month, day) {
var date = new Date(year,month-1,day)
return date.getDay();
}
this.clearDate = clearDate;
function clearDate() {
dateField.value = '';
hide();
}
this.setDate = setDate;
function setDate(year, month, day) {
if (dateField) {
if (month < 10) {month = "0" + month;}
if (day < 10) {day = "0" + day;}
var dateString = month+"-"+day+"-"+year;
dateField.value = dateString;
hide();
}
return;
}
this.changeMonth = changeMonth;
function changeMonth(change) {
currentMonth += change;
currentDay = 0;
if(currentMonth > 12) {
currentMonth = 1;
currentYear++;
} else if(currentMonth < 1) {
currentMonth = 12;
currentYear--;
}
calendar = document.getElementById(calendarId);
calendar.innerHTML = calendarDrawTable();
}
this.changeYear = changeYear;
function changeYear(change) {
currentYear += change;
currentDay = 0;
calendar = document.getElementById(calendarId);
calendar.innerHTML = calendarDrawTable();
}
function getCurrentYear() {
var year = new Date().getYear();
if(year < 1900) year += 1900;
return year;
}
function getCurrentMonth() {
return new Date().getMonth() + 1;
}
function getCurrentDay() {
return new Date().getDate();
}
function calendarDrawTable() {
var dayOfMonth = 1;
var validDay = 0;
var startDayOfWeek = getDayOfWeek(currentYear, currentMonth, dayOfMonth);
var daysInMonth = getDaysInMonth(currentYear, currentMonth);
var css_class = null; //CSS class for each day
var table = "<table cellspacing='0' cellpadding='0' border='0'>";
table = table + "<tr class='header'>";
table = table + " <td colspan='2' class='previous'><a href='javascript
:changeCalendarControlMonth(-1);'><</a> <a href='javascript
:changeCalendarControlYear(-1);'>«</a></td>";
table = table + " <td colspan='3' class='title'>" + months[currentMonth-1] + "<br>" + currentYear + "</td>";
table = table + " <td colspan='2' class='next'><a href='javascript
:changeCalendarControlYear(1);'>&r aquo;</a> <a href='javascript
:changeCalendarControlMonth(1);'>& gt;</a></td>";
table = table + "</tr>";
table = table + "<tr><th>D</th><th>L</th><th>M</th><th>M</th><th>J</th><th>V</th><th>S</th></tr>";
for(var week=0; week < 6; week++) {
table = table + "<tr>";
for(var dayOfWeek=0; dayOfWeek < 7; dayOfWeek++) {
if(week == 0 && startDayOfWeek == dayOfWeek) {
validDay = 1;
} else if (validDay == 1 && dayOfMonth > daysInMonth) {
validDay = 0;
}
if(validDay) {
if (dayOfMonth == selectedDay && currentYear == selectedYear && currentMonth == selectedMonth) {
css_class = 'current';
} else if (dayOfWeek == 0 || dayOfWeek == 6) {
css_class = 'weekend';
} else {
css_class = 'weekday';
}
table = table + "<td><a class='"+css_class+"' href=\"javascript
:setCalendarControlDate("+current Year+","+currentMonth+","+dayOfMonth+")\">"+dayOfM onth+"</a></td>";
dayOfMonth++;
} else {
table = table + "<td class='empty'> </td>";
}
}
table = table + "</tr>";
}
table = table + "<tr class='header'><th colspan='7' style='padding: 3px;'><a href='javascript
:clearCalendarControl();'>Borrar</a> | <a href='javascript
:hideCalendarControl();'>Cerrar</a></td></tr>";
table = table + "</table>";
return table;
}
this.show = show;
function show(field) {
can_hide = 0;
// If the calendar is visible and associated with
// this field do not do anything.
if (dateField == field) {
return;
} else {
dateField = field;
}
if(dateField) {
try {
var dateString = new String(dateField.value);
var dateParts = dateString.split("-");
selectedMonth = parseInt(dateParts[0],10);
selectedDay = parseInt(dateParts[1],10);
selectedYear = parseInt(dateParts[2],10);
} catch(e) {}
}
if (!(selectedYear && selectedMonth && selectedDay)) {
selectedMonth = getCurrentMonth();
selectedDay = getCurrentDay();
selectedYear = getCurrentYear();
}
currentMonth = selectedMonth;
currentDay = selectedDay;
currentYear = selectedYear;
if(document.getElementById){
calendar = document.getElementById(calendarId);
calendar.innerHTML = calendarDrawTable(currentYear, currentMonth);
setProperty('display', 'block');
var fieldPos = new positionInfo(dateField);
var calendarPos = new positionInfo(calendarId);
var x = fieldPos.getElementLeft();
var y = fieldPos.getElementBottom();
setProperty('left', x + "px");
setProperty('top', y + "px");
if (document.all) {
setElementProperty('display', 'block', 'CalendarControlIFrame');
setElementProperty('left', x + "px", 'CalendarControlIFrame');
setElementProperty('top', y + "px", 'CalendarControlIFrame');
setElementProperty('width', calendarPos.getElementWidth() + "px", 'CalendarControlIFrame');
setElementProperty('height', calendarPos.getElementHeight() + "px", 'CalendarControlIFrame');
}
}
}
this.hide = hide;
function hide() {
if(dateField) {
setProperty('display', 'none');
setElementProperty('display', 'none', 'CalendarControlIFrame');
dateField = null;
}
}
this.visible = visible;
function visible() {
return dateField
}
this.can_hide = can_hide;
var can_hide = 0;
}
var calendarControl = new CalendarControl();
function showCalendarControl(textField) {
// textField.onblur = hideCalendarControl;
calendarControl.show(textField);
}
function clearCalendarControl() {
calendarControl.clearDate();
}
function hideCalendarControl() {
if (calendarControl.visible()) {
calendarControl.hide();
}
}
function setCalendarControlDate(year, month, day) {
calendarControl.setDate(year, month, day);
}
function changeCalendarControlYear(change) {
calendarControl.changeYear(change);
}
function changeCalendarControlMonth(change) {
calendarControl.changeMonth(change);
}
document.write("<iframe id='CalendarControlIFrame' src='javascript
:false;' frameBorder='0' scrolling='no'></iframe>");
document.write("<div id='CalendarControl'></div>");
Lo que necesito es que no se puedan seleccionar dias posteriores al dia actual, es decir que si estamos 20 de febrero no pueda seleccionar el 21 de febrero, el 22 y asi consecutivamente, espero disculpen mi ignorancia pero no se mucho de javascript y espero que me puedan ayudar 
Mejorar Calendario Javascript 
