Is there any date/time where this function could break?

Question

I've not seen anything like this before. This is part of a function that returns the expected answer (a series of five dates)... sometimes. For example, it's been run at 6am, and the result is sometimes incorrect: one of the five dates, either at the first or last, can be missing. Other times, it's fine. Same code, run just a few hours later.

I know working with dates can be a lot more complicated than it first appears, but this has stumped me. I can only hope my inexperience with the DateTime objects is to blame.

    $start = new \DateTime(date("Y-m-d", strtotime("-1 day"))); 
    $end = new \DateTime(date("Y-m-d", strtotime("-5 days"))); 
    $diff = $end->diff($start);
    $interval = \DateInterval::createFromDateString('-1 day');
    $period = new \DatePeriod($start, $interval, $diff->days); 

    foreach($period as $date) {
        echo $date->format("Y-m-d"); // Sometimes first or last date will be missing
    }

So for example, if the code is run between 2020-07-05 00:00:00 and 2020-07-05 23:59:59, it should return the last five dates:

2020-07-04
2020-07-03
2020-07-02
2020-07-01
2020-06-30

I've run the code with various date/times manually, and I cannot recreate the bug... and yet it happens once every few days in production.

This is just vanilla PHP, but it is being run as part of a Laravel project, should that factor into things. (The app timezone is set to "Europe/London".)

Answer 1

I'm not keen on how you're defining $start and $end. If I'm not mistaken, if the server clock happens to tick to the next second between the two variables being defined, then your interval will be 3 days, 23 hours, 59 minutes, 59 seconds - instead of exactly 4 days. This messes up your definition of $diff->days to be 3 instead of 4, leading to a missing date.

I would suggest a different approach here. Specifically, start with the current date, and subtract a day the number of times you want - since this appears to be hard-coded to 5.

$date = new DateTime();
$interval = new DateInterval("P1D");
for( $i=0; $i<5; $i++) {
    $date->sub($interval);
    echo $date->format("Y-m-d")."\n";
}

That $i<5 can, of course, be refactored to $i < DAYS for some appropriate constant definition, to avoid the "magic number" and allow for changing in future development.