从 JavaScript 中的周数计算日期

Question

在知道周数和年份的情况下，如何在 JavaScript 中计算日期？对于第 20 周和 2013 年，我想获得 2013-05-16。 我正在尝试这样：

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

Answer 1

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

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

这里使用simple week definition，表示 2013 年的第 20 周是 5 月 14 日。

计算给定ISO8601 week 的开始日期（始终是星期一）

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;
}

结果：2013年第20周是5月13日，可以确认here。

Answer 2

抱歉，这有点冗长，但此解决方案也会在计算周数后返回星期日。它结合了我从几个不同地方看到的答案：

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 3

ISO 周

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 4

根据周数获取日期范围

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 5

2019 版（如果有人想计算一周的开始日期）

Elle 的回答非常正确，但有一个例外： 如果您想获得一周的开始日期 - 这对您没有帮助，因为 Elle 的算法根本不介意星期的开始日期。

原因如下：

Elle的解决方案

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

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

这里您基本上只是计算从年初（1 月 1 日）算起的天数。这意味着您的一年从哪一天开始并不重要：星期一、星期日或星期五 - 不会有任何区别。

要设置一周的开始日期（例如星期一），您只需：

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 6

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);

Answer 7

ISO 星期日期（例如 2017-W32-3）到正常日期：

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 8

@adius 功能对我有用，但我为正在寻找简单复制粘贴解决方案的人做了一些改进：

/**
 * get date by week number
 * @param  {Number} year 
 * @param  {Number} week 
 * @param  {Number} day of week (optional; default = 0 (Sunday)) 
 * @return {Date}      
 */

  function weekToDate(year, week, weekDay = 0) {
    const getZeroBasedIsoWeekDay = date => (date.getDay() + 6) % 7;
    const getIsoWeekDay = date => getZeroBasedIsoWeekDay(date) + 1;
    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);
  }


// test: 
const x = [
  {year: 2001, week: 10},
  {year: 2019, week: 7},
  {year: 2020, week: 5},
  {year: 2020, week: 1},
  {year: 2020, week: 12}, 
  {year: 2020, week: 2},
  {year: 2020, week: 40},
  {year: 2021, week: 4},
  {year: 2021, week: 3},
  {year: 2021, week: 2},
  {year: 2032, week: 1}
]

for (let i = 0; i < x.length; i++) {
  let date = weekToDate(x[i].year, x[i].week, 0);
  let d = new Date(date).toLocaleDateString('en-US', { 
    year: 'numeric',
    month: 'long',
    day: 'numeric' 
  })

  console.log(d);
}

Answer 9

这是用于获取当年一周的第一个星期一（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 10

我刚刚创建了这个：

//Returns monday and saturday of the week
function getWeekRange(weekNo,YearNo) {
    let firstDayofYear = new Date(yearNo, 0, 1);

    if (firstDayofYear.getDay() > 4)  {
        let weekStart = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 8);
        let weekEnd = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 8 + 5);
        return { startDay: weekStart, endDay: weekEnd }
    }
    else {
        let weekStart = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 1);
        let weekEnd = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 1 + 5);
        return { startDay: weekStart, endDay: weekEnd }
    }
}