如何从数字中获取 PHP 中的特定日期？

Question

Basically, I have a date set to today. From that date, I want to get the next and previous 20th day of the month. Basically, starting with 07/10/2020 (date of the post in d/m/Y format), I should end up with a date/timestamp to 20/09/2020 (previous 20th) and 20/10/2020 (next 20th).

I tried a few things and ended up with:

$ld_next_str = date("d/m/Y", strtotime("20 day this month"));
$ld_next_dt = DateTime::createFromFormat("d/m/Y", $ld_next_str);

$ld_prev_str = date("d/m/Y", strtotime("20 day last month"));
$ld_prev_dt = DateTime::createFromFormat("d/m/Y", $ld_prev_str);

But turns out this : "20 day this month" just adds 20 days to the current day so right now (on the seventh) it returns the 27th.

Knowing this, I figured I could probably just do something like subtracting the current day to that date (27 - 7 = 20) but I feel like there's probably a simpler way to do it, using strtotime without any extra steps.

Thanks in advance!

PS : I know you can get some days using "first" / "second" / "third" etc... but (I think) it doesn't go any higher than "twelfth", because I tried "twentieth" and that didn't work...

Answer 1

The formula for the previous 20th is:

mktime(0, 0, 0, $month - (1 - floor($day / 20)), 20, $year))

The formula for the next 20th is:

mktime(0, 0, 0, $month + floor($day / 20), 20, $year))

As a demo:

for ($month = 1; $month <= 12; $month++) {
    for ($day = 1; $day <= 30; $day += 5) {
        $prev = mktime(0, 0, 0, $month - (1 - floor($day / 20)), 20, 2020);
        $next = mktime(0, 0, 0, $month + floor($day / 20), 20, 2020);
        echo "$day/$month/2020: ", date('d/m/Y', $prev), ', ', date('d/m/Y', $next), PHP_EOL;
    }
}

Outputs:

1/1/2020: 20/12/2019, 20/01/2020
6/1/2020: 20/12/2019, 20/01/2020
11/1/2020: 20/12/2019, 20/01/2020
16/1/2020: 20/12/2019, 20/01/2020
21/1/2020: 20/01/2020, 20/02/2020
26/1/2020: 20/01/2020, 20/02/2020
1/2/2020: 20/01/2020, 20/02/2020
6/2/2020: 20/01/2020, 20/02/2020
11/2/2020: 20/01/2020, 20/02/2020
16/2/2020: 20/01/2020, 20/02/2020
21/2/2020: 20/02/2020, 20/03/2020
...

Answer 2

simply that:

$lastM = date("20/m/Y", strtotime("-1 month"));
$thisM = date("20/m/Y", time());
$nextM = date("20/m/Y", strtotime("+1 month"));

echo "$lastM <br /> $thisM <br /> $nextM";

Answer 3

Solution with an external class (a DateTime extension) that supports the cron syntax.

The string "0 0 20 * *" specifies the 20th of every month and every weekday at 00:00. The nextCron() and previousCron() methods are used to determine the previous/next point in time based on a specific date.

$cronStr = "0 0 20 * *";  //every 20th 00:00

//source on https://github.com/jspit-de/dt
$next20 = dt::create('now')->nextCron($cronStr);
echo $next20;  //2020-10-20 00:00:00

$prev20 = dt::create('now')->previousCron($cronStr);
echo $prev20;  //2020-09-20 00:00:00

Outputs for the execution on October 8, 2020.