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

Here's my implementation, which supports fractional days. The concept is simple: get the unix timestamp of midnight on the first day of the year, then multiply the desired day by the number of milliseconds in a day.

/**
 * Converts day of the year to a unix timestamp
 * @param {Number} dayOfYear 1-365, with support for floats
 * @param {Number} year (optional) 2 or 4 digit year representation. Defaults to
 * current year.
 * @return {Number} Unix timestamp (ms precision)
 */
function dayOfYearToTimestamp(dayOfYear, year) {
  year = year || (new Date()).getFullYear();
  var dayMS = 1000 * 60 * 60 * 24;

  // Note the Z, forcing this to UTC time.  Without this it would be a local time, which would have to be further adjusted to account for timezone.
  var yearStart = new Date('1/1/' + year + ' 0:0:0 Z');

  return yearStart + ((dayOfYear - 1) * dayMS);
}

// usage

// 2015-01-01T00:00:00.000Z
console.log(new Date(dayOfYearToTimestamp(1, 2015)));

// support for fractional day (for satellite TLE propagation, etc)
// 2015-06-29T12:19:03.437Z
console.log(new Date(dayOfYearToTimestamp(180.51323423, 2015)).toISOString);

Answer 2

You have a few options;

If you're using a standard format, you can do something like:

new Date(dateStr);

If you'd rather be safe about it, you could do:

var date, timestamp;
try {
    timestamp = Date.parse(dateStr);
} catch(e) {}
if(timestamp)
    date = new Date(timestamp);

or simply,    

new Date(Date.parse(dateStr));

Or, if you have an arbitrary format, split the string/parse it into units, and do:

new Date(year, month - 1, day)

Example of the last:

var dateStr = '28/10/2010'; // uncommon US short date
var dateArr = dateStr.split('/');
var dateObj = new Date(dateArr[2], parseInt(dateArr[1]) - 1, dateArr[0]);

Answer 3

The shortest possible way is to create a new date object with the given year, January as month and your day of the year as date:

const date = new Date(2017, 0, 365);

console.log(date.toLocaleDateString());

As for setDate the correct month gets calculated if the given date is larger than the month's length.

Answer 4

this also works ..

function to2(x) { return ("0"+x).slice(-2); }
function formatDate(d){
    return d.getFullYear()+"-"+to2(d.getMonth()+1)+"-"+to2(d.getDate());
    }
document.write(formatDate(new Date(2016,0,257)));

prints "2016-09-13"

which is correct as 2016 is a leaap year. (see calendars here: http://disc.sci.gsfc.nasa.gov/julian_calendar.html )

Answer 5

"I want to take a day of the year and convert to an actual date using the Date object."

After re-reading your question, it sounds like you have a year number, and an arbitrary day number (e.g. a number within 0..365 (or 366 for a leap year)), and you want to get a date from that.

For example:

dateFromDay(2010, 301); // "Thu Oct 28 2010", today ;)
dateFromDay(2010, 365); // "Fri Dec 31 2010"

If it's that, can be done easily:

function dateFromDay(year, day){
  var date = new Date(year, 0); // initialize a date in `year-01-01`
  return new Date(date.setDate(day)); // add the number of days
}

You could add also some validation, to ensure that the day number is withing the range of days in the year supplied.

Answer 6

// You might need both parts of it-

Date.fromDayofYear= function(n, y){
    if(!y) y= new Date().getFullYear();
    var d= new Date(y, 0, 1);
    return new Date(d.setMonth(0, n));
}
Date.prototype.dayofYear= function(){
    var d= new Date(this.getFullYear(), 0, 0);
    return Math.floor((this-d)/8.64e+7);
}

var d=new Date().dayofYear();
//
alert('day#'+d+' is '+Date.fromDayofYear(d).toLocaleDateString())


/*  returned value: (String)
day#301 is Thursday, October 28, 2010
*/