Javascript：计算给定年份的月数

Question

I have a HTML page with 3 dropdowns for the month, day and year and I was wondering if there was a way to populate the month drop down properly depending on the month and year.

I haven't done this before on the client side, but it looks like a lot of controls like the jQuery DatePicker are doing that behind the scenes.

Answer 1

As far as I know, there's no (neat) built-in function for that. I wrote this once:

// note that month is 0-based, like in the Date object. Adjust if necessary.
function getNumberOfDays(year, month) {
    var isLeap = ((year % 4) == 0 && ((year % 100) != 0 || (year % 400) == 0));
    return [31, (isLeap ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
}

Answer 2

You can play with date objects:

var monthStart = new Date(year, month, 1);
var monthEnd = new Date(year, month + 1, 1);
var monthLength = (monthEnd - monthStart) / (1000 * 60 * 60 * 24)

Arithmetic with Date objects gives a number of milliseconds.

This will even work for December; the Date constructor handles out-of-range arguments by wrapping around.

Note that month is zero-based (it must be between 0 and 11 )

Answer 3

Copy from another post: Get number days in a specified month using javascript?

//Month is 1 based
function daysInMonth(month,year) {
return new Date(year, month, 0).getDate();
}

//July
daysInMonth(7,2009); //31
//February
daysInMonth(2,2009); //28
daysInMonth(2,2008); //29

All the credits to @c_harm, really great solution

Answer 4

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

function daysinMonthfromInput(month,year){
    return (new Date(year,month-1,1)).daysinMonth();
}

alert(daysinMonthfromInput(2,2011));

Answer 5

Here's the one liner. Assuming you are saying January=1, February=2 etc..(being normal) Here's the leap year example:

var y = 2012;
var m = 2;
var daysInMonth = new Date(y,m,1,-1).getDate();

Answer 6

I am using this approach in my current project and found that I needed correct for round off errors. So instead of using monthLength in my code, I had to use this instead:

monthLength.toFixed(0)

For example if I have an object in which I am storing a text date field, it may look like this:

obj.month = theMonths[mm - 1] + " " + monthLength.toFixed(0) + ", " + obj.year;

Answer 7

You can actually use this:

var curdate = new Date(); DaysMonth = 32 - new Date(curdate.getYear(), curdate.getMonth(), 32).getDate();

;)

Answer 8

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

function daysinMonthfromInput  (month, year) {
     return (new Date(year, month - 1, 1)).daysinMonth();
};
function fillallday (elem, month, year) {
     var options = null;
     var elementExists = document.getElementById(elem);

     if (elementExists != null) {

         this.removeOptions(elementExists);
         var opt = document.createElement('option');
         opt.value = "";
         opt.innerHTML = "---Day---";
         elementExists.appendChild(opt);
         if (month != "") {
             if (typeof (year) === "undefined") {
                 year = new Date().getFullYear();
             }
             if (year == "") {
                 year = new Date().getFullYear();
             }
             var days = daysinMonthfromInput(month, year);
             for (var i = 1; i <= days; i++) {
                 var opt = document.createElement('option');
                 opt.value = i;
                 opt.innerHTML = i;
                 elementExists.appendChild(opt);
             }
         }
     }

 }