how can I convert day of year to date in javascript?

Question

I want to take a day of the year and convert to an actual date using the Date object. Example: day 257 of 1929, how can I go about doing this?

Answer 1

If you always want a UTC date:

function getDateFromDayOfYear (year, day) {
  return new Date(Date.UTC(year, 0, day))
}

console.log(getDateFromDayOfYear(2020, 1)) // 2020-01-01T00:00:00.000Z
console.log(getDateFromDayOfYear(2020, 305)) // 2020-10-31T00:00:00.000Z
console.log(getDateFromDayOfYear(2020, 366)) // 2020-12-31T00:00:00.000Z

Answer 2

Here is a function that takes a day number, and returns the date object

optionally, it takes a year in YYYY format for parameter 2. If you leave it off, it will default to current year.

var getDateFromDayNum = function(dayNum, year){

    var date = new Date();
    if(year){
        date.setFullYear(year);
    }
    date.setMonth(0);
    date.setDate(0);
    var timeOfFirst = date.getTime(); // this is the time in milliseconds of 1/1/YYYY
    var dayMilli = 1000 * 60 * 60 * 24;
    var dayNumMilli = dayNum * dayMilli;
    date.setTime(timeOfFirst + dayNumMilli);
    return date;
}

OUTPUT

// OUTPUT OF DAY 232 of year 1995

var pastDate = getDateFromDayNum(232,1995)
console.log("PAST DATE: " , pastDate);

PAST DATE: Sun Aug 20 1995 09:47:18 GMT-0400 (EDT)

Answer 3

If I understand your question correctly, you can do that from the Date constructor like this

new Date(year, month, day, hours, minutes, seconds, milliseconds)

All arguments as integers