我有一个月的周数，并且想用 PHP 获取该周内的第一个日期

Question

假设我有以下给定信息：$year = 2020;  $month = 7;  $week_nr_of_month = 2;  $first_date_in_that_week = ?;我想知道该月该周内的第一个日期。所以在我目前的情况下输出应该是 2020-07-06我该如何计算这个？谢谢。

Answer 1

我有这样的事情：

<?php

function myDate(int $year, int $month, int $week): string {

  $firstDayDate = DateTime::createFromFormat('j-n-Y', '1-'.$month.'-'.$year);

  $firstDayDate->setTime(0,0);

  $firstMondayDate = (clone $firstDayDate)->modify('first monday of this month');

  $modificator = $week - 2;

  if ($firstMondayDate->format('d') === $firstDayDate->format('d')) {

    $modificator++;

  }

  if ($modificator < 0) {

    return $firstDayDate->format('Y-m-d');

  }

  $wantedWeekDate = (clone $firstMondayDate)->add(new DateInterval('P'.($modificator*7).'D'));

  

  return $wantedWeekDate->format('Y-m-d');

}

var_dump(

myDate(2020, 7, 1),

myDate(2020, 7, 2),

myDate(2020, 7, 3),

myDate(2020, 7, 4),

myDate(2020, 7, 5),

myDate(2020, 7, 6),

myDate(2020, 8, 1),

myDate(2020, 8, 2),

myDate(2020, 8, 3),

myDate(2020, 8, 4),

myDate(2020, 8, 5),

myDate(2020, 8, 6),

myDate(2014, 12, 1),

myDate(2014, 12, 2),

myDate(2014, 12, 3)

);

测试输出：

string(10) "2020-07-01"

string(10) "2020-07-06"

string(10) "2020-07-13"

string(10) "2020-07-20"

string(10) "2020-07-27"

string(10) "2020-08-03"

string(10) "2020-08-01"

string(10) "2020-08-03"

string(10) "2020-08-10"

string(10) "2020-08-17"

string(10) "2020-08-24"

string(10) "2020-08-31"

string(10) "2014-12-01"

string(10) "2014-12-08"

string(10) "2014-12-15"

Answer 2

这是我的解决方案：

function firstDayOfWeekInMonth($year, $month, $week)

{

    $firstOfMonth = strtotime("$year-$month-1");

    $format = 'Y-m-d';

    if ($week == 1) {

        return date($format, $firstOfMonth);

    } else {

        $secondsInOneDay = 60 * 60 * 24;

        $secondInOneWeek = $secondsInOneDay * 7;

        $dayNumberOfFirstOfMonth = date('w', $firstOfMonth); //eg Monday = 1, Tuesday = 2, etc.

        $offsetToStartOfWeek = ($dayNumberOfFirstOfMonth - 1) * $secondsInOneDay; // eg Monday is 0 days from Monday, Tuesday is 1 day from Monday etc.

        $startOfWeek1 = $firstOfMonth - $offsetToStartOfWeek; //could be in previous month

        $StartOfWeekN = $startOfWeek1 + ($week - 1) * $secondsInOneWeek; // week-2 is 1-week ahead of week-1's Monday

        return date($format, $StartOfWeekN);

    }

}

该算法使用自 Unix 纪元以来的秒数。这是一个整数，因此所有计算都是通过整数相加或相乘来完成的，这应该相当快。日期和整数之间的转换仅在函数开始和结束时发生一次。

顺便说一句，当我指的是如此多不同的周/月的开始时，我发现为一周的开始找到有意义的名称是一个很大的挑战。

为了进行比较，我使用了与布莱希的答案相同的测试：

var_dump(

firstDayOfWeekInMonth(2020, 7, 1),

firstDayOfWeekInMonth(2020, 7, 2),

firstDayOfWeekInMonth(2020, 7, 3),

firstDayOfWeekInMonth(2020, 7, 4),

firstDayOfWeekInMonth(2020, 7, 5),

firstDayOfWeekInMonth(2020, 7, 6),

firstDayOfWeekInMonth(2020, 8, 1),

firstDayOfWeekInMonth(2020, 8, 2),

firstDayOfWeekInMonth(2020, 8, 3),

firstDayOfWeekInMonth(2020, 8, 4),

firstDayOfWeekInMonth(2020, 8, 5),

firstDayOfWeekInMonth(2020, 8, 6),

firstDayOfWeekInMonth(2014, 12, 1),

firstDayOfWeekInMonth(2014, 12, 2),

firstDayOfWeekInMonth(2014, 12, 3)

);

string(10) "2020-07-01"

string(10) "2020-07-06"

string(10) "2020-07-13"

string(10) "2020-07-20"

string(10) "2020-07-27"

string(10) "2020-08-03"

string(10) "2020-08-01"

string(10) "2020-08-03"

string(10) "2020-08-10"

string(10) "2020-08-17"

string(10) "2020-08-24"

string(10) "2020-08-31"

string(10) "2014-12-01"

string(10) "2014-12-08"

string(10) "2014-12-15"