# What is the best way to determine the number of days in a month with JavaScript?

I've been using this function but I'd like to know what's the most efficient and accurate way to get it.

``````function daysInMonth(iMonth, iYear) {
return 32 - new Date(iYear, iMonth, 32).getDate();
}
``````

If you call this function often, it may be useful to cache the value for better performance.

Here is caching version of FlySwat's answer:

``````var daysInMonth = (function() {
var cache = {};
return function(month, year) {
var entry = year + '-' + month;

if (cache[entry]) return cache[entry];

return cache[entry] = new Date(year, month, 0).getDate();
}
})();
``````

Some answers (also on other questions) had leap-year problems or used the Date-object. Although javascript's `Date object` covers approximately 285616 years (100,000,000 days) on either side of January 1 1970, I was fed up with all kinds of unexpected date inconsistencies across different browsers (most notably year 0 to 99). I was also curious how to calculate it.

So I wrote a simple and above all, small algorithm to calculate the correct (Proleptic Gregorian / Astronomical / ISO 8601:2004 (clause 4.3.2.1), so year `0` exists and is a leap year and negative years are supported) number of day's for a given month and year.
It uses the short-circuit bitmask-modulo leapYear algorithm (slightly modified for js) and common mod-8 month algorithm.

Note that in `AD/BC` notation, year 0 AD/BC does not exist: instead year `1 BC` is the leap-year!
IF you need to account for BC notation then simply subtract one year of the (otherwise positive) year-value first!! (Or subtract the year from `1` for further year-calculations.)

``````function daysInMonth(m, y){
return m===2?y&3||!(y%25)&&y&15?28:29:30+(m+(m>>3)&1);
}``````
``````<!-- example for the snippet -->
<input type="text" value="enter year" onblur="
for( var r='', i=0, y=+this.value
; 12>i++
; r+= 'Month: ' + i + ' has ' + daysInMonth(i, y) + ' days<br>'
);
this.nextSibling.innerHTML=r;
" /><div></div>``````

Note, months must be 1-based!

Note, this is a different algorithm then the magic number lookup I used in my Javascript calculate the day of the year (1 - 366) answer, because here the extra branch for the leap-year is only needed for February.

To take away confusion I would probably make the month string based as it is currently 1 based.

``````function daysInMonth(month,year) {
monthNum =  new Date(Date.parse(month +" 1,"+year)).getMonth()+1
return new Date(year, monthNum, 0).getDate();
}

daysInMonth('feb', 2015)
//28

daysInMonth('feb', 2008)
//29
``````

One-liner direct computation (no Date object):

``````function daysInMonth(m, y) {//m is 1-based, feb = 2
return 31 - (--m ^ 1? m % 7 & 1:  y & 3? 3: y % 25? 2: y & 15? 3: 2);
}

console.log(daysInMonth(2, 1999)); // February in a non-leap year
console.log(daysInMonth(2, 2000)); // February in a leap year``````

Variation with 0-based months:

``````function daysInMonth(m, y) {//m is 0-based, feb = 1
return 31 - (m ^ 1? m % 7 & 1:  y & 3? 3: y % 25? 2: y & 15? 3: 2);
}
``````

If you want the number of days in the current month of a Date object, consider the following method:

``````Date.prototype.getNumberOfDaysInMonth = function(monthOffset) {
if (monthOffset !== undefined) {
return new Date(this.getFullYear(), this.getMonth()+monthOffset, 0).getDate();
} else {
return new Date(this.getFullYear(), this.getMonth(), 0).getDate();
}
}
``````

Then you can run it like this:

``````var myDate = new Date();
myDate.getNumberOfDaysInMonth(); // Returns 28, 29, 30, 31, etc. as necessary
myDate.getNumberOfDaysInMonth(); // BONUS: This also tells you the number of days in past/future months!
``````

In a single line:

``````// month is 1-12
function getDaysInMonth(year, month){
return month == 2 ? 28 + (year % 4 == 0 ? (year % 100 == 0 ? (year % 400 == 0 ? 1 : 0) : 1):0) : 31 - (month - 1) % 7 % 2;
}
``````

Here is goes

``````new Date(2019,2,0).getDate(); //28
new Date(2020,2,0).getDate(); //29
``````
``````function numberOfDays(iMonth, iYear) {
var myDate = new Date(iYear, iMonth + 1, 1);  //find the fist day of next month
var newDate = new Date(myDate - 1);  //find the last day
return newDate.getDate();         //return # of days in this month
}
``````

Considering leap years:

``````function (year, month) {
var isLeapYear = ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0);

return [31, (isLeapYear ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
}
``````

Perhaps not the most elegant solution, but easy to understand and maintain; and, it's battle-tested.

``````function daysInMonth(month, year) {
var days;
switch (month) {
case 1: // Feb, our problem child
var leapYear = ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
days = leapYear ? 29 : 28;
break;
case 3: case 5: case 8: case 10:
days = 30;
break;
default:
days = 31;
}
return days;
},
``````

``````function daysInMonth (month, year) { // Use 1 for January, 2 for February, etc.
return new Date(year, month, 0).getDate();
}

console.log(daysInMonth(2, 1999)); // February in a non-leap year.
console.log(daysInMonth(2, 2000)); // February in a leap year.``````

Day 0 is the last day in the previous month. Because the month constructor is 0-based, this works nicely. A bit of a hack, but that's basically what you're doing by subtracting 32.

With moment.js you can use daysInMonth() method:

``````moment().daysInMonth(); // number of days in the current month
moment("2012-02", "YYYY-MM").daysInMonth() // 29
moment("2012-01", "YYYY-MM").daysInMonth() // 31
``````

May be bit over kill when compared to selected answer :) But here it is:

``````function getDayCountOfMonth(year, month) {
if (month === 3 || month === 5 || month === 8 || month === 10) {
return 30;
}

if (month === 1) {
if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
return 29;
} else {
return 28;
}
}

return 31;
};

console.log(getDayCountOfMonth(2020, 1));``````

I found the above code over here: https://github.com/ElemeFE/element/blob/dev/src/utils/date-util.js

``````function isLeapYear(year) {
return ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0);
};

const getDaysInMonth = function (year, month) {
return [31, (isLeapYear(year) ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
};

console.log(getDaysInMonth(2020, 1));``````

I found the above code over here: https://github.com/datejs/Datejs/blob/master/src/core.js

If you are going to pass a date variable this may helpful

``````const getDaysInMonth = date =>
new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();

daysInThisMonth = getDaysInMonth(new Date());

console.log(daysInThisMonth);``````

ES6 syntax

``````const d = (y, m) => new Date(y, m, 0).getDate();
``````

returns

``````console.log( d(2020, 2) );
// 29

console.log( d(2020, 6) );
// 30
``````