Correct Day, for Month

How can I change the day when the user select a month?
When they select Feb ->29 days, select May -> 31 days.

Like what facebook does.

// we will suppose that you have a <select> for the year and one for the <month>
//
var form = document.YourFormName;
var yr = form.yearSelect[form.yearSelect.selectedIndex].value; // any valid 4 digit year
var mn = form.yearSelect[form.monthSelect.selectedIndex].value; // 1 - 12

var firstOfNextMonth = new Date( yr, mn, 1 ); // in JS month goes 0 to 11, so this works
var lastOfGivenMonth = new Date( firstOfNextMonth.getTime() - 24 * 60 *60 * 1000 );

var lastDayOfGivenMonth = lastOfGivenMonth.getDate();
...


Note: without the year, you can't get the right number of days for February. Though of course it would work for any other month with any year.

Dates, unlike months and weekdays are not 0 based.

The last day of a month is the zeroth day of the next month.

Date.prototype.daysinMonth=function(){
var d= new Date(this.getFullYear(), this.getMonth()+1, 0);
return d.getDate();
}

var d=new Date();
alert(d.daysinMonth());

var M= ['Jan','Feb','Mar','Apr','May','Jun','Jul',
'Aug','Sep','Oct','Nov','Dec'],
i= 0, y= 2012, A= [], d= new Date(y, 0, 1);
while(i<12){
A[i]= M[i]+' has '+d.daysinMonth()+' days';
d.setMonth(++i);
}
alert('Days in each Month of '+y+'\n\t'+A.join('\n\t'))

Check out the OO Dropdown Date Picker script from my sig. Scroll down to post #15 for the latest version.

Never tried using 0 for last-day-of-prior month. Had a hunch it would work, but hadn't tested it so gave the next-best answer.

It works with DateSerial() in VB/VBS/Access, by the by, if anybody cares.