Add days to Javascript Date object, and also increment month

Question

Looking over the previous questions and answers it appeared this should work :

var palindrome = new Date('2011-11-11');
var december = new Date('2011-11-11');

december.setDate(palindrome.getDate()+20); 
//should be december, but in fact loops back over to Nov 1st)

my jsFiddle

is there a simple way to ensure that months are incremented correctly, or have I missed something obvious ?

Answer 1

You could do it like this:

var dayOffset = 20;
var millisecondOffset = dayOffset * 24 * 60 * 60 * 1000;
december.setTime(december.getTime() + millisecondOffset); 

Answer 2

The getMonth() call returns a value between 0 and 11, where 0 is January and 11 is December, so 10 means November. You need to increment the value by 1 when using it in a string. If you simply output it as a string you'll see that it has the correct date. Note I also had to change the starting date format. It didn't seem to like 2011-11-11 so I made it 11/11/2011 . http://jsfiddle.net/9HLSW/

Answer 3

Your code is correct, however you are converting it to a string wrongly .

getMonth() starts with 0 as January, and ends with 11 as December. So all you need to do is add 1 to the month like this:

alert(endDate.getFullYear() + "-" + (endDate.getMonth()+1) +"-"+ endDate.getDate());   

Notice the additional brackets - cos you are performing a math operation while concatenating strings. You won't want to end up with " 101 " as the month.

To see whether you got the date correct, use endDate.toDateString() to display the date in a fully qualified name (ie: January - December).

alert(endDate.toDateString());

For more info on the Date object, check out this section in w3schools