如何在Javascript中获取每个月的指定日期？

Question

我需要找到本月、上月和下月的具体日期。

例如，日期设置为每月 31 日，我希望得到的日期是 2018 年 2 月 28 日、2018 年 3 月 31 日和 2018 年 4 月 30 日。对于那些没有 31 的日期，它会变成前一天。

最后生成 2 个周期，2018-02-28 到 2018-03-29，2018-03-30 到 2018-04-31。 2月和31以下的月份不知道怎么处理。

var d = new Date();
var tyear = d.getFullYear(); //2018
var tmonth = d.getMonth();  //2  
new Date(2018, tmonth-1, 31);//output 2018-03-02 not what I wanted

Answer 1

一个简单的算法是在原始日期的基础上增加月份，如果新日期错误，则将其设置为上个月的最后一天。保持原始日期值不变有助于，例如

/* @param {Date} start - date to start
** @param {number} count - number of months to generate dates for
** @returns {Array} monthly Dates from start for count months
*/
function getMonthlyDates(start, count) {
  var result = [];
  var temp;
  var year = start.getFullYear();
  var month = start.getMonth();
  var startDay = start.getDate();
  for (var i=0; i<count; i++) {
    temp = new Date(year, month + i, startDay);
    if (temp.getDate() != startDay) temp.setDate(0);
    result.push(temp);
  }
  return result;
}

// Start on 31 Jan in leap year
getMonthlyDates(new Date(2016,0,31), 4).forEach(d => console.log(d.toString()));
// Start on 31 Jan not in leap year
getMonthlyDates(new Date(2018,0,31), 4).forEach(d => console.log(d.toString()));

// Start on 30 Jan
getMonthlyDates(new Date(2018,0,30), 4).forEach(d => console.log(d.toString()));
// Start on 5 Jan
getMonthlyDates(new Date(2018,0,5), 4).forEach(d => console.log(d.toString()));

Answer 2

我认为您将需要一个包含 12 个数字的数组。每个数字是每个月的天数，数组中的数字按顺序排列（第一个数字是 31，因为 1 月有 31 天，第二个数字是 28 或 29 的 2 月）等。然后你会得到月份数您的输入日期并在数组中查看与月份数 +/- 1 对应的数字。

然后，您需要根据当月的天数构造上个月和下个月的日期。

查看 cmets 内联：

let daysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

document.getElementById("date").addEventListener("input", function(){
  console.clear();
  
  // Create new Date based on value in date picker
  var selectedDate = new Date(this.value + 'T00:00');

  var year = selectedDate.getYear();

  // Determine if it is a leap year (Feb has 29 days) and update array if so.
  if (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)) {
    daysInMonths[1] = 29;
  }

  var selectedDateMonth = selectedDate.getMonth();
     
  // Get previous month number (if current month is January, get December)
  let prevMonth = selectedDateMonth > 0 ? selectedDateMonth - 1 : 11;
  
  let prevMonthDate = null;
  
  // If selected date is last day of month...
  if(selectedDate.getDate() === daysInMonths[selectedDateMonth]){
    // Create new date that takes the selected date and subtracts the correct amount of
    // days from it based on a lookup in the array.
    var newDate1 = new Date(selectedDate.getTime());
    prevMonthDate = 
     new Date(newDate1.setDate(selectedDate.getDate() - daysInMonths[selectedDateMonth]));
  } else {
    // Create a new date that is last month and one day earlier
    var newDate2 = new Date(selectedDate.getTime());
    prevMonthDate = 
      new Date(new Date(newDate2.setDate(selectedDate.getDate() - 1))
        .setMonth(selectedDate.getMonth() - 1));
  }
  
  // Get next month (if current month is December, get January
  let nextMonth = selectedDateMonth < 11 ? selectedDateMonth + 1 : 0;  
  
  let nextMonthDate = null;
  
  // Same idea for next month, but add instead of subtract.
  
  // If selected date is last day of month...
  if(selectedDate.getDate() === daysInMonths[selectedDateMonth]){
    var newDate3 = new Date(selectedDate.getTime());
    nextMonthDate = 
     new Date(newDate3.setDate(selectedDate.getDate() + daysInMonths[selectedDateMonth + 1]));
  } else {
    var newDate4 = new Date(selectedDate.getTime());
    nextMonthDate = new Date(new Date(newDate4.setDate(selectedDate.getDate() + 1)).setMonth(selectedDate.getMonth() + 1));
  }  

  console.log("Last month date: " + prevMonthDate.toLocaleDateString());
  console.log("Next month date: " + nextMonthDate.toLocaleDateString());  
});

<p>Pick a date: <input type="date" id="date"></p>

Answer 3

使用这种方法：

Javascript Date Object – Adding and Subtracting Months

来自作者

尝试前进到下个月或返回上个月时，Javascript Date() 对象存在小问题。

例如，如果您将日期设置为 2018 年 10 月 31 日并添加了一个月，您可能会认为新日期为 2018 年 11 月 30 日，因为 11 月 31 日不存在。然而，事实并非如此。

Javascript 会自动将您的 Date 对象提前到 12 月 1 日。此功能在大多数情况下非常有用（例如，为日期添加天数、确定一个月中的天数或是否是闰年），但不适用于添加/减去月份。我在下面汇总了一些扩展 Date() 对象的函数：nextMonth() 和 prevMonth()。

function prevMonth() {
  var thisMonth = this.getMonth();
  this.setMonth(thisMonth - 1);
  if (this.getMonth() != thisMonth - 1 && (this.getMonth() != 11 || (thisMonth == 11 && this.getDate() == 1)))
    this.setDate(0);
}

function nextMonth() {
  var thisMonth = this.getMonth();
  this.setMonth(thisMonth + 1);
  if (this.getMonth() != thisMonth + 1 && this.getMonth() != 0)
    this.setDate(0);
}

Date.prototype.nextMonth = nextMonth;
Date.prototype.prevMonth = prevMonth;

var today = new Date(2018, 2, 31); //<----- March 31st, 2018

var prevMonth = new Date(today.getTime());
prevMonth.prevMonth();
console.log("Previous month:", prevMonth);

console.log("This month:", today)

var nextMonth = new Date(today.getTime());
nextMonth.nextMonth();
console.log("Next month:", nextMonth);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Answer 4

日期和时区是 JS 的一大难题，因此接受挑战。

我分两步分解：
- 计算上个月和下个月的天数
- 与选定的日期进行比较并选择最小的数字

包含测试用例

function createUTCDate(year, month, day) {
  return new Date(Date.UTC(year, month, day));
}

function splitDate(date) {
  return {
    year: date.getUTCFullYear(),
    month: date.getUTCMonth(),
    day: date.getUTCDate()
  };
}

function numberOfDaysInMonth(year, month) {
  return new Date(year, month + 1, 0).getDate();
}

function dateNextMonth(dateObj) {
  const daysNextMonth = numberOfDaysInMonth(dateObj.year, dateObj.month + 1);
  const day = Math.min(daysNextMonth, dateObj.day);
  return createUTCDate(dateObj.year, dateObj.month + 1, day);
}

function datePreviousMonth(dateObj) {
  const daysPrevMonth = numberOfDaysInMonth(dateObj.year, dateObj.month - 1);
  const day = Math.min(daysPrevMonth, dateObj.day);
  return createUTCDate(dateObj.year, dateObj.month - 1, day);
}

const log = console.log;

function print(dateString) {
  const date = new Date(dateString);
  const dateObj = splitDate(date);
  log("Previous: ", datePreviousMonth(dateObj).toISOString());
  log("Selected: ", date.toISOString());
  log("Next: ", dateNextMonth(dateObj).toISOString());
  log("--------------");
}

const testCases = [
  "2018-03-01 UTC",
  "2018-03-31 UTC",
  "2018-01-01 UTC",
  "2018-12-31 UTC"
];

testCases.forEach(print);

请注意，new Date(xxx + " UTC") 的 hack 不符合规范，仅用于测试目的。结果可能因浏览器而异。 您应该选择一种输入格式并相应地构建您的日期。

Answer 5

我通过连接字符串以一种愚蠢的方式处理它

let daysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
let months = ["01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12"];

var target = nexttarget = lasttarget = "29"; //target day

if (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)) {
    daysInMonths[1] = 29;
}

function findLastDay(target, month){
    if(target > daysInMonths[month]){
        target = daysInMonths[month];
    }
    return target;
}

然后

var d = new Date();
var year = d.getFullYear();
var month = d.getMonth();

target = findLastDay(target, month);

var this_month = year+"-"+months[month]+"-"+target;
console.log(this_month);//2018-03-29

// next month
if(month == 11){
   nextmonth = 0;
   nextyear = year + 1;
}else{
   nextmonth = month+1;
   nextyear = year;
}

nexttarget = findLastDay(nexttarget, nextmonth);

var next_month = nextyear+"-"+months[nextmonth]+"-"+nexttarget;
console.log(next_month);//2018-04-29

//last month
if(month == 0){
   lastmonth = 11;
   lastyear = year - 1;
}else{
   lastmonth = month - 1;
   lastyear = year;
}

lasttarget = findLastDay(lasttarget, lastmonth);

var last_month = lastyear+"-"+months[lastmonth]+"-"+lasttarget;
console.log(last_month);//2018-02-28

Answer 6

在最好的情况下，日期处理很棘手。不要自己这样做。使用Moment.js。

var target = 31;
var today = moment().date(target).calendar();
//  today == '03/31/2018'

var nextMonth =  moment().date(target).add(1, 'month').calendar();
// nextMonth == '04/30/2018'

var lastMonth = moment().date(target).subtract(1, 'month').calendar()
// lastMonth == '02/28/2018'