How to convert week number to date in javascript

Question

I tried to find some work, but I was not lucky. Any ideas?

Examples:

Week, 1, 2001 => 2001-01-01

Week, 26, 2007 => 2007-06-01

Answer 1

Since Kevin’s code does not meet the ISO 8601 standard (the first day of the first week of the year should be on Monday), I fixed it and received it (also check it on jsfiddle ):

function firstDayOfWeek(week, year) { if (year==null) { year = (new Date()).getFullYear(); } var date = firstWeekOfYear(year), weekTime = weeksToMilliseconds(week), targetTime = date.getTime() + weekTime; return date.setTime(targetTime); } function weeksToMilliseconds(weeks) { return 1000 * 60 * 60 * 24 * 7 * (weeks - 1); } function firstWeekOfYear(year) { var date = new Date(); date = firstDayOfYear(date,year); date = firstWeekday(date); return date; } function firstDayOfYear(date, year) { date.setYear(year); date.setDate(1); date.setMonth(0); date.setHours(0); date.setMinutes(0); date.setSeconds(0); date.setMilliseconds(0); return date; } /** * Sets the given date as the first day of week of the first week of year. */ function firstWeekday(firstOfJanuaryDate) { // 0 correspond au dimanche et 6 correspond au samedi. var FIRST_DAY_OF_WEEK = 1; // Monday, according to iso8601 var WEEK_LENGTH = 7; // 7 days per week var day = firstOfJanuaryDate.getDay(); day = (day === 0) ? 7 : day; // make the days monday-sunday equals to 1-7 instead of 0-6 var dayOffset=-day+FIRST_DAY_OF_WEEK; // dayOffset will correct the date in order to get a Monday if (WEEK_LENGTH-day+1<4) { // the current week has not the minimum 4 days required by iso 8601 => add one week dayOffset += WEEK_LENGTH; } return new Date(firstOfJanuaryDate.getTime()+dayOffset*24*60*60*1000); } function assertDateEquals(effectiveDate, expectedDate, description) { if ((effectiveDate==null ^ expectedDate==null) || effectiveDate.getTime()!=expectedDate.getTime()) { console.log("assert failed: "+description+"; effective="+effectiveDate+", expected="+expectedDate); } } function assertEquals(effectiveValue, expectedValue, description) { if (effectiveValue!=expectedValue) { console.log("assert failed: "+description+"; effective="+effectiveValue+", expected="+expectedValue); } } // expect the first day of year to be a monday for (var i=1970; i<2050; i++) { assertEquals(firstWeekOfYear(i).getDay(), 1, "first day of year "+i+" must be a monday"); // 1=Monday } // assert some future first day of first week of year; source: http://www.epochconverter.com/date-and-time/weeknumbers-by-year.php assertDateEquals(firstWeekOfYear(2013), new Date(Date.parse("Dec 31, 2012")), "2013"); assertDateEquals(firstWeekOfYear(2014), new Date(Date.parse("Dec 30, 2013")), "2014"); assertDateEquals(firstWeekOfYear(2015), new Date(Date.parse("Dec 29, 2014")), "2015"); assertDateEquals(firstWeekOfYear(2016), new Date(Date.parse("Jan 4, 2016")), "2016"); assertDateEquals(firstWeekOfYear(2017), new Date(Date.parse("Jan 2, 2017")), "2017"); assertDateEquals(firstWeekOfYear(2018), new Date(Date.parse("Jan 1, 2018")), "2018"); assertDateEquals(firstWeekOfYear(2019), new Date(Date.parse("Dec 31, 2018")), "2019"); assertDateEquals(firstWeekOfYear(2020), new Date(Date.parse("Dec 30, 2019")), "2020"); assertDateEquals(firstWeekOfYear(2021), new Date(Date.parse("Jan 4, 2021")), "2021"); assertDateEquals(firstWeekOfYear(2022), new Date(Date.parse("Jan 3, 2022")), "2022"); assertDateEquals(firstWeekOfYear(2023), new Date(Date.parse("Jan 2, 2023")), "2023"); assertDateEquals(firstWeekOfYear(2024), new Date(Date.parse("Jan 1, 2024")), "2024"); assertDateEquals(firstWeekOfYear(2025), new Date(Date.parse("Dec 30, 2024")), "2025"); assertDateEquals(firstWeekOfYear(2026), new Date(Date.parse("Dec 29, 2025")), "2026"); console.log("All assertions done."); 

I included test cases for some dates to verify that the first day of the first week of the year is Monday, and checked some dates based on http://www.epochconverter.com/date-and-time/weeknumbers-by-year.php

Answer 2

Someone might be interested in a more meaningful version:

 function firstDayOfWeek (year, week) { // Jan 1 of 'year' var d = new Date(year, 0, 1), offset = d.getTimezoneOffset(); // ISO: week 1 is the one with the year first Thursday // so nearest Thursday: current date + 4 - current day number // Sunday is converted from 0 to 7 d.setDate(d.getDate() + 4 - (d.getDay() || 7)); // 7 days * (week - overlapping first week) d.setTime(d.getTime() + 7 * 24 * 60 * 60 * 1000 * (week + (year == d.getFullYear() ? -1 : 0 ))); // daylight savings fix d.setTime(d.getTime() + (d.getTimezoneOffset() - offset) * 60 * 1000); // back to Monday (from Thursday) d.setDate(d.getDate() - 3); return d; } 

Answer 3

Look at this fiddle . Firstly, he receives the first week of the specified year. This takes into account that in accordance with ISO 8601, the first week of the year is the first week containing the environment. Then it adds weeks to the received date and returns the result.

 function firstDayOfWeek(week, year) { var date = firstWeekOfYear(year), weekTime = weeksToMilliseconds(week), targetTime = weekTime + date.getTime(); return date.setTime(targetTime); } 

Answer 4

No, there is no function for this. You must do it manually. Follow this way .

Answer 5

I took the original Kevin idea with some tweaks as the source code returns milliseconds. Here you are:

 var d = firstDayOfWeek(9, 2013); console.log(d.format("yyyy-MM-dd")); ////////////////////////////// Main Code ////////////////////////////// function firstDayOfWeek(week, year) { if (typeof year !== 'undefined') { year = (new Date()).getFullYear(); } var date = firstWeekOfYear(year), weekTime = weeksToMilliseconds(week), targetTime = date.getTime() + weekTime - 86400000; var result = new Date(targetTime) return result; } function weeksToMilliseconds(weeks) { return 1000 * 60 * 60 * 24 * 7 * (weeks - 1); } function firstWeekOfYear(year) { var date = new Date(); date = firstDayOfYear(date,year); date = firstWeekday(date); return date; } function firstDayOfYear(date, year) { date.setYear(year); date.setDate(1); date.setMonth(0); date.setHours(0); date.setMinutes(0); date.setSeconds(0); date.setMilliseconds(0); return date; } function firstWeekday(date) { var day = date.getDay(), day = (day === 0) ? 7 : day; if (day > 3) { var remaining = 8 - day, target = remaining + 1; date.setDate(target); } return date; }