javascript calculate date from week number

How i can calculate the date in javascript knowing weeknumber and the year? For week number 20 and year 2013 to obtain 5/16/2013 I am trying so:

Date.prototype.dayofYear = function() {        
    var d = new Date(this.getFullYear(), 0, 0);        
    return Math.floor((enter code here this - d) / 8.64e + 7); 
}

Answers:

Answer

ISO Weeks

function weekDateToDate (year, week, day) {
  const firstDayOfYear = new Date(year, 0, 1)
  const days = 2 + day + (week - 1) * 7 - firstDayOfYear.getDay()
  return new Date(year, 0, days)
}
Answer

Sorry if this is a little verbose but this solution will also return the Sunday after calculating the week number. It combines answers I've seen from a couple different places:

function getSundayFromWeekNum(weekNum, year) {
    var sunday = new Date(year, 0, (1 + (weekNum - 1) * 7));
    while (sunday.getDay() !== 0) {
        sunday.setDate(sunday.getDate() - 1);
    }
    return sunday;
}
Answer

Get date range according to week number

Date.prototype.getWeek = function () {
    var target  = new Date(this.valueOf());
    var dayNr   = (this.getDay() + 6) % 7;
    target.setDate(target.getDate() - dayNr + 3);
    var firstThursday = target.valueOf();
    target.setMonth(0, 1);
    if (target.getDay() != 4) {
        target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
    }
    return 1 + Math.ceil((firstThursday - target) / 604800000);
}

function getDateRangeOfWeek(weekNo){
    var d1 = new Date();
    numOfdaysPastSinceLastMonday = eval(d1.getDay()- 1);
    d1.setDate(d1.getDate() - numOfdaysPastSinceLastMonday);
    var weekNoToday = d1.getWeek();
    var weeksInTheFuture = eval( weekNo - weekNoToday );
    d1.setDate(d1.getDate() + eval( 7 * weeksInTheFuture ));
    var rangeIsFrom = eval(d1.getMonth()+1) +"/" + d1.getDate() + "/" + d1.getFullYear();
    d1.setDate(d1.getDate() + 6);
    var rangeIsTo = eval(d1.getMonth()+1) +"/" + d1.getDate() + "/" + d1.getFullYear() ;
    return rangeIsFrom + " to "+rangeIsTo;
};

getDateRangeOfWeek(10)
"3/6/2017 to 3/12/2017"

getDateRangeOfWeek(30)
"7/24/2017 to 7/30/2017"
Answer

This is for getting the first monday of a week in the current year (ISO 8601):

function getFirstMondayOfWeek(weekNo) {
    var firstMonday = new Date(new Date().getFullYear(), 0, 4, 0, 0, 0, 0);

    while (firstMonday.getDay() != 1) {
        firstMonday.setDate(firstMonday.getDate() - 1);
    }
    if (1 <= weekNo && weekNo <= 52)
        return firstMonday.setDate(firstMonday.getDate() + 7 * (weekNo - 1));

    firstMonday.setDate(firstMonday.getDate() + 7 * (weekNo - 1));
    if (weekNo = 53 && firstMonday.getDate() >= 22 && firstMonday.getDate() <= 28)
        return firstMonday;
    return null;
}
Answer

2019 version (if anyone wants to calculate the date of week's start)

The answer of Elle is pretty much right, but there is one exception: if you want to get a date of start of the week - it won't help you because Elle's algorithm doesn't mind the day, from which the week starts of, at all.

And here is why:

Elle's solution

function getDateOfWeek(w, y) {
  var d = (1 + (w - 1) * 7); // 1st of January + 7 days for each week

  return new Date(y, 0, d);
}

Here you basically just calculating the amount of days from the start of the year (1st January). This means that it doesn't matter from which day your year starts of: Monday, Sunday or Friday - it will not make any difference.

To set the day, from which your week will start of (let's say Monday, for example), you just need to:

function getDateOfWeek(w, y) {
  let date = new Date(y, 0, (1 + (w - 1) * 7)); // Elle's method
  date.setDate(date.getDate() + (1 - date.getDay())); // 0 - Sunday, 1 - Monday etc
  return date
}
Answer

ISO week date (e.g. 2017-W32-3) to normal date:

const getZeroBasedIsoWeekDay = date => (date.getDay() + 6) % 7
const getIsoWeekDay = date => getZeroBasedIsoWeekDay(date) + 1

function weekDateToDate(year, week, weekDay) {
  const zeroBasedWeek = week - 1
  const zeroBasedWeekDay = weekDay - 1
  let days = (zeroBasedWeek * 7) + zeroBasedWeekDay

  // Dates start at 2017-01-01 and not 2017-01-00
  days += 1

  const firstDayOfYear = new Date(year, 0, 1)
  const firstIsoWeekDay = getIsoWeekDay(firstDayOfYear)
  const zeroBasedFirstIsoWeekDay = getZeroBasedIsoWeekDay(firstDayOfYear)

  // If year begins with W52 or W53
  if (firstIsoWeekDay > 4) days += 8 - firstIsoWeekDay
  // Else begins with W01
  else days -= zeroBasedFirstIsoWeekDay

  return new Date(year, 0, days)
}

const expectedAndActual = [
  [new Date('2007-01-01'), weekDateToDate(2007,  1, 1)],
  [new Date('2015-11-24'), weekDateToDate(2015, 48, 2)],
  [new Date('2015-12-31'), weekDateToDate(2015, 53, 4)],
  [new Date('2016-01-03'), weekDateToDate(2015, 53, 7)],
  [new Date('2017-01-01'), weekDateToDate(2016, 52, 7)],
  [new Date('2017-01-02'), weekDateToDate(2017,  1, 1)],
  [new Date('2017-05-07'), weekDateToDate(2017, 18, 7)],
  [new Date('2018-12-31'), weekDateToDate(2019,  1, 1)],
]

expectedAndActual
  .forEach(value => {
    const expected = value[0].toISOString()
    const actual = value[1].toISOString()
    const isEqual = actual === expected
    console.assert(isEqual, '%s !== %s', actual, expected)
    console.log(actual, '===', expected)
  })

Answer
function getDateOfWeek(w, y) {
    var d = (1 + (w - 1) * 7); // 1st of January + 7 days for each week

    return new Date(y, 0, d);
}

This uses the simple week definition, meaning the 20th week of 2013 is May 14.

To calculate the date of the start of a given ISO8601 week (which will always be a Monday)

function getDateOfISOWeek(w, y) {
    var simple = new Date(y, 0, 1 + (w - 1) * 7);
    var dow = simple.getDay();
    var ISOweekStart = simple;
    if (dow <= 4)
        ISOweekStart.setDate(simple.getDate() - simple.getDay() + 1);
    else
        ISOweekStart.setDate(simple.getDate() + 8 - simple.getDay());
    return ISOweekStart;
}

Result: the 20th week of 2013 is May 13, which can be confirmed here.

Answer
function getDateOfWeek(weekNumber,year){
    //Create a date object starting january first of chosen year, plus the number of days in a week multiplied by the week number to get the right date.
    return new Date(year, 0, 1+((weekNumber-1)*7));
}
var myDate = getDateOfWeek(20,2013);

Tags

Recent Questions

Top Questions

Home Tags Terms of Service Privacy Policy DMCA Contact Us

©2020 All rights reserved.