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

"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 2

// 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 */