Javascript Date.UTC() function is off by a month?

Question

I was playing around with Javascript creating a simple countdown clock when I came across this strange behavior:

var a = new Date(), 
now = a.getTime(),
then = Date.UTC(2009,10,31),
diff = then - now,
daysleft = parseInt(diff/(24*60*60*1000));
console.log(daysleft );

The days left is off by 30 days.

What is wrong with this code?

Edit: I changed the variable names to make it more clear.

Answer 1

As Eric said, this is due to months being listed as 0-11 range.

This is a common behavior - same is true of Perl results from localtime(), and probably many other languages.

This is likely originally inherited from Unix's localtime() call. (do "man localtime")

The reason is that days/years are their own integers, while months (as a #) are indexes of an array, which in most languages - especially C where the underlying call is implemented on Unix - starts with 0.

Answer 2

It's an old question but this is still a problem today (or a feature as some might say - and they are wrong).

JS is zero-based month, why? Because.

That means the months range from 0-11 (only the months, the others are normal)

How can you fix this? Add a month, obviously, BUUUUT:

Don't do this :

let date: Date = new Date();
date.setMonth(date.getMonth() + 1);

Why you might ask? Because it won't work as expected, Date in JS is terrible.

You have to make a ... let's call it not so beautiful function to translate the JS date to a normal date

formatJsDateToNormalDate(Date date): string | null {
  if(date !== null) {
        const realMonth: number = date.getMonth() + 1;
        let month: string = (realMonth < 10) ? '0' + realMonth : String(realMonth);
        let day: string = (date.getDate() < 10) ? '0' + date.getDate() : String(date.getDate());
        
        return [date.getFullYear(), month, day].join('-');
  } else {
    return null;
}

Again, if you ask me this is the equivalent of hammering a screw, it's not the right way, but there is no right way here, it's a bug that has been going on for 27 years and more to come.