DateInterval issues with negative directions

[cjhewett]

Description

I am going to put a few connected issues I have noticed around DateInterval and DatePeriod into a single report:

1: DateInterval->invert is "readonly"

DateInterval:::__construct() only allows positive $duration e.g. P1D not P-1D (I appreciate this is the correct behaviour as it is based around the ISO8601 which has no concept of negative periods). However... the DateInterval->invert which allows the period to be reversed can only be changed by manually setting that property and the docs explicitly say:

The available properties listed below depend on PHP version, and should be considered as readonly.

It would be good to be able to change this property safely.

2: DateInterval::createFromDateString allows negative periods

DateInterval::createFromDateString('-1 day') is accepted but this sets DateInterval->d = -1 rather than using the ::invert property. This is inconsistent with the always-positive periods which __construct allows.

I would expect it to:

• Reject mixed positive and negative period values, what would +1 year -6 month even mean?
• Accept if only negative periods passed but then set ::invert = 1 and the properties of ::y..f as +ve

3: DateInterval::createFromDateString('-1 day') leads to infinite loop in DatePeriod

$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-01'),
    \DateInterval::createFromDateString('-1 day'),
    new \DateTimeImmutable('2000-01-05'),
);

echo iterator_count($datePeriod); // stuck in infinite loop

4: DateInterval::createFromDateString('-1 day') still doesn't work if start and end reversed in DatePeriod

$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-05'),
    \DateInterval::createFromDateString('-1 day'),
    new \DateTimeImmutable('2000-01-01'),
);

echo iterator_count($datePeriod); // 0 (but would expect 4)

5: DatePeriod doesn't respect DateInterval->invert

$dateInterval = new \DateInterval('P1D');
$dateInterval->invert = 1;

# will not go backwards from high to low dates
$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-05'),
    $dateInterval,
    new \DateTimeImmutable('2000-01-01'),
);

echo iterator_count($datePeriod); // 0 (but would expect 4)

# incorrectly goes forward from low to high dates
$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-01'),
    $dateInterval,
    new \DateTimeImmutable('2000-01-05'),
);

echo iterator_count($datePeriod); // 4 (but would expect 0)

PHP Version

8.3.114

Operating System

Rocky 8

[hormus]

-1 day Is relative formats.
https://www.php.net/manual/en/datetime.formats.php#datetime.formats.relative

Alternative syntax for relative formats:

<?php

//Wrong DateInterval first monday of january 2020, it progress year (+1, +2, etc) not have start or end this class.
$timezone = new DateTimeZone( "UTC" );
$fmj = DateInterval::createFromDateString( 'first monday of next month' );
$period = new DatePeriod( new DateTime( '2020-01-01 00:00:00.000000', $timezone ), $fmj, new DateTime( '2022-01-11 00:00:00.000000', $timezone ) );
$y = 0;
$c[0] = $period;
foreach ( $period as $dt ) {
$c[$y] = $period;
    $display[$y] = $dt->format( "l Y-m-d H:i:s" );
		//stop total_recurrence
        $y = $y + 1;
		if( $y == 25 ) {
            $y = 24;
			break;
		}
}
$var = new DateTime( 'first monday of January 2020', $timezone);
$display[0] = 'this relative date for DateTime class ' . ($var->format( "l Y-m-d H:i:s" ));
var_dump( $display, $fmj, $c[$y]);
?>

Old bug. 79104

1. You can set "DateInterval->invert" to known code. DatePeriod relative formats are not known.

If -1 day, use recurrence from targetobject

<?php

$timezone = new DateTimeZone( "UTC" );
$fmj = DateInterval::createFromDateString('-1 day');
$period = new DatePeriod( new DateTime( '2020-01-01 00:00:00.000000', $timezone ), $fmj, 4 );
$y = 0;
$c[0] = $period;
$display[0] = '0';
$var = $fmj;
foreach ( $period as $dt ) {
$c[$y] = $period;
    $display[$y] = $dt->format( "l Y-m-d H:i:s" );
		//stop total_recurrence
        $y = $y + 1;
		if( $y == 5 ) {
            $y = 4;
            $var = $fmj->d;
			break;
		}
}
var_dump($display, $var, $c[$y]);
?>

Expected result PHP 8.2 >=:

array(5) {
  [0]=>
  string(29) "Wednesday 2020-01-01 00:00:00"
  [1]=>
  string(27) "Tuesday 2019-12-31 00:00:00"
  [2]=>
  string(26) "Monday 2019-12-30 00:00:00"
  [3]=>
  string(26) "Sunday 2019-12-29 00:00:00"
  [4]=>
  string(28) "Saturday 2019-12-28 00:00:00"
}
int(-1)
object(DatePeriod)#3 (7) {
  ["start"]=>
  object(DateTime)#5 (3) {
    ["date"]=>
    string(26) "2020-01-01 00:00:00.000000"
    ["timezone_type"]=>
    int(3)
    ["timezone"]=>
    string(3) "UTC"
  }
  ["current"]=>
  object(DateTime)#8 (3) {
    ["date"]=>
    string(26) "2019-12-28 00:00:00.000000"
    ["timezone_type"]=>
    int(3)
    ["timezone"]=>
    string(3) "UTC"
  }
  ["end"]=>
  NULL
  ["interval"]=>
  object(DateInterval)#6 (10) {
    ["y"]=>
    int(0)
    ["m"]=>
    int(0)
    ["d"]=>
    int(-1)
    ["h"]=>
    int(0)
    ["i"]=>
    int(0)
    ["s"]=>
    int(0)
    ["f"]=>
    float(0)
    ["invert"]=>
    int(0)
    ["days"]=>
    bool(false)
    ["from_string"]=>
    bool(false)
  }
  ["recurrences"]=>
  int(5)
  ["include_start_date"]=>
  bool(true)
  ["include_end_date"]=>
  bool(false)
}

[cjhewett]

-1 day Is relative formats.

I do appreciate why createFromDateString is accepting "-1 day", my point is that this should lead to a DateInterval which looks like:

DateInterval {
  [d] => 1
  [invert] => 1
  ...
}

but it is actually producing

DateInterval {
  [d] => -1
  [invert] => 0
  ...
}

which is inconsistent with ISO8601 which uses non negative periods.

You can set "DateInterval->invert" to known code

You can set it, but the manual explicitly warns that the property is "considered as readonly".

If -1 day, use recurrence from targetobject

This indeed does work as expected:

$datePeriod = new \DatePeriod(
  new \DateTimeImmutable('2000-01-05'),
  \DateInterval::createFromDateString('-1 day'),
  3,
);

echo iterator_count($datePeriod); // 4

echo implode(', ', array_map(
  fn ($dt) => $dt->format('Y-m-d'),
  iterator_to_array($datePeriod),
)); // 2000-01-05, 2000-01-04, 2000-01-03, 2000-01-02

But that is even better evidence that the other constructor flavour should be able to go backwards between a high $start and low $end

public __construct(
  DateTimeInterface $start,
  DateInterval $interval,
  DateTimeInterface $end,
  int $options = 0
)

[derickr]

It would be good to be able to change this property safely. (DateInterval->invert).
…
I do appreciate why createFromDateString is accepting "-1 day", my point is that this should lead to a DateInterval which looks like: …

You can't always invert relative strings (what does inverting next monday mean?). So that's why it doesn't do that, and shouldn't.
These properties need to be made properly readonly, for which there is a PR: #13050 — it isn't as trivial, as it would introduce a BC break.

what would +1 year -6 month even mean?

It means: Add 1 to the year number, and subtract 6 from the month number. And then normalise, effectively doing +6 months, but with some shenanigans around the 29th/30th/31st of the month.

3: DateInterval::createFromDateString('-1 day') leads to infinite loop in DatePeriod.

Yes, because it never reaches the end cut-off date, just like in for ($i = 0; $i <= 10; $i--).

4: DateInterval::createFromDateString('-1 day') still doesn't work if start and end reversed in DatePeriod

DatePeriod is meant for iterating forwards only. This probably should be documented, and/or an exception thrown for.

5: DatePeriod doesn't respect DateInterval->invert

You said yourself: "the docs explicitly say: The available properties listed below depend on PHP version, and should be considered as readonly." — so why are you setting it and expecting it to work?

But that is even better evidence that the other constructor flavour should be able to go backwards between a high $start and low $end

The DatePeriod constructor can't do that, as it doesn't know that the DateInterval subtracts time.

---

There are most definitely ways on how to improve DatePeriod/DateInterval, but beyond not checking whether the end date (if set) is after the start date, and making the properties of DateInterval read only, there is not an actionable bug right now.

[cjhewett]

These properties need to be made properly readonly

An 'inverted' DateInterval can be created via a diff of 2 dates:

$dateInterval = (new \DateTimeImmutable('2000-01-02'))->diff(
   new \DateTimeImmutable('2000-01-01')
);

var_dump($dateInterval->invert); // true

Why should this be allowed, but then have no way to create the same inverted interval using the actual DateInterval class constructor?

If its truly made readonly then I put forward that there needs to be a ::setInvert(bool $invert) method.

It means: Add 1 to the year number, and subtract 6 from the month number. And then normalise, effectively doing +6 months, but with some shenanigans around the 29th/30th/31st of the month.

That is not an ISO 8601 compatible duration though, that is a set of multiple durations which have to be applied to move from one point in time to another.

My point is an "interval" should be a single directed duration of time, it can be positive (meaning x after y) or negative (meaning x before y), but not elements of positive and negative at the same time, at that point its not a single quantity defining a duration but multiple durations which need to be applied (and is therefore incompatible with the concept of a single DateInterval).

Java has a Duration class which stores its duration as a single positive or negative quantity of seconds.

Allowing ::createFromDateString() to specify anything but a simple directional duration has corrupted the core concept behind this DateInterval class. I would put forward that the from_string & date_string properties are just a bodge which effectively holds the equivalent of an array of durations while pretending to be a single one.

How can 'last day of next month' be considered a "duration"? This is a modifier which can be applied to a date, but not a duration.

You said yourself: "the docs explicitly say: The available properties listed below depend on PHP version, and should be considered as readonly." — so why are you setting it and expecting it to work?

I was only using a manually set invert for the simplicity of the reproduction example. If I use a legally created DateInterval result from the diff of 2 dates which has invert=true, the reproduction case still stands:

$dateInterval = (new \DateTimeImmutable('2000-01-02'))->diff(
   new \DateTimeImmutable('2000-01-01')
);

# will not go backwards from high to low dates
$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-05'),
    $dateInterval,
    new \DateTimeImmutable('2000-01-01'),
);

echo iterator_count($datePeriod); // 0 (but would expect 4)

# incorrectly goes forward from low to high dates
$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-01'),
    $dateInterval,
    new \DateTimeImmutable('2000-01-05'),
);

echo iterator_count($datePeriod); // 4 (but would expect 0)