我有一个月的周数，并想在 PHP 中获得该周内的第一个日期

Question

Assuming I have the following given information:

$year = 2020; 
$month = 7; 

$week_nr_of_month = 2; 

$first_date_in_that_week = ?; 

I want to know the first date within that week number of month. So in my current situation the output should be the 2020-07-06

How can I calculate this? Thank you.

Answer 1

I have something like this:

<?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)
);

Tested output:

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

Here is my solution:

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);
    }
}

This algorithm uses the number of seconds since the Unix epoch. This is an integer so all the calculations are done by adding or multiplying integers which should be quite fast. The conversion between a date and an integer only happens once at the start and once at the end of the function.

As an aside, I found finding meaningful names for the start of the week when I was referring to so many different starts of weeks/months quite a challenge.

For comparison I've used the same tests as in blahy's answer:

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"