Re: Add ISO 8601 date format constant with millisecond precision

Hey Théo On 29.08.26 18:04, Théo Attali wrote:

Hi Andreas, Thank you for your feedback. I agree that a format constant containing a literal Z is not ideal because the format string cannot enforce UTC. A userland formatter is safer because it can normalize the value before formatting it. After considering this, I think an instance method may be a better fit than a new format constant:

     $date->toISOString();

The method could normalize the instant to UTC and return exactly:

     1970-01-01T00:00:00.000Z

It would not modify the original object, which is particularly important for DateTimeImmutable. I also found that Carbon provides:

     $date->toIso8601ZuluString('millisecond');

However, that requires an external dependency. My motivation is to provide a dependency-free way to produce a common ISO 8601 UTC representation used when exchanging timestamps between PHP, JavaScript, Java, Go, and other ecosystems. I searched for an existing PHP issue or RFC proposing this exact method but found only the older, inactive DateTime::__toString() RFC, which addressed a different problem. Would you consider a UTC-normalizing instance method a more appropriate direction for this issue than adding a format constant?

Well... PHP already *has* a dependency-free way to format any datetime-object into an ISO string: echo $anyDateTimeImmutable

    ->setTimezone(new DateTimeZone('UTC'))
    ->format('Y-m-d\\TH:i:s.vP');

If you want to avoid having to write that string all the time, feel free to do something like Interface IsoDateTime {

    const WITH_MILLISECONDS = 'Y-m-d\\TH:i:s.vP';

    const JUST_SECONDS = 'Y-m-d\\TH:i:sP';

} and then call echo $anyDateTimeImmutable

    ->setTimezone(new DateTimeZone('UTC'))
    ->format(IsoDateTimeInterface::WITH_MILLISECONDS);

That works since PHP5.3 Anything else is adding convenience at a cost. And for me (but I am just one person here on the list) that cost (limited availability to older versions, increasingly polluted constants, very specific use-case, possibly broken output due to wrong timezone) outweighs the benfits. Just my 0.02€ Cheers Andreas

Best regards, Théo Attali El sáb, 29 ago 2026 a las 12:36, Andreas Heigl (<andreas@heigl.org>) escribió:

Hey Théo On 29.08.26 11:47, Théo Attali wrote:

Hi ! Sorry derick for the duplicated email but I was not registered. This is my first contribution, so I would appreciate any help on the proposal and on following the project's contribution process. I opened issue #23491 for a possible addition to ext/date:

      https://github.com/php/php-src/issues/23491

I would appreciate feedback before preparing an implementation. PHP currently provides DATE_RFC3339_EXTENDED, which includes millisecond precision. For a UTC DateTime, it produces:

      1970-01-01T00:00:00.000+00:00

Some applications need the equivalent representation with the ISO 8601

UTC

designator Z:

      1970-01-01T00:00:00.000Z

This is also the format produced by JavaScript's Date.prototype.toISOString():

      new Date(0).toISOString();
      // "1970-01-01T00:00:00.000Z"

Would a predefined format for this representation be appropriate for ext/date? One possible API would be:

      DATE_ISO8601_MILLISECONDS_UTC
      DateTimeInterface::ISO8601_MILLISECONDS_UTC

with the format string:

      "Y-m-d\\TH:i:s.v\\Z"

For example:

      $date = new DateTimeImmutable('@0');
      echo $date->setTimezone(new DateTimeZone('UTC'))
          ->format(DATE_ISO8601_MILLISECONDS_UTC);

      // 1970-01-01T00:00:00.000Z

The name and API shape are open for discussion. Another option would be

an

offset-preserving format:

      DATE_ISO8601_MILLISECONDS
      DateTimeInterface::ISO8601_MILLISECONDS

using:

      "Y-m-d\\TH:i:s.vP"

I understand that date format constants only define formatting; they do

not

change the timezone of the DateTime object. A format containing a literal Z would therefore require the caller to normalize the object to UTC first. For context, issue #14593 discusses the interpretation of the Z suffix

when

parsing DateTime values:

      https://github.com/php/php-src/issues/14593

That issue concerns parsing and timezone representation, whereas this proposal concerns predefined formatting constants. I would appreciate feedback on: 1. Whether a millisecond-precision format is useful in addition to

     DATE_RFC3339_EXTENDED.

2. Whether an offset-preserving format or a UTC-suffix format is

preferable.

3. Whether the proposed names follow the preferred ext/date convention. 4. Whether an RFC is required before implementation. Thank you, Théo Attali

I see some serious issues with the idea of adding the format with the \\Z at the end as you already described. It would require a prerequisite that can'T be enforced and that will therefore cause a lot of confusion. To ease that confusion I would recommend anyone that has that explicit requirement to create a Formatter - perhaps tehr eeven is one already available that in essence just does something like this: class Formatter {

      public static function formatIso(DateTimeImmutable $date): string
      {
         return $date->setTimezone(new

DateTimeZone('UTC'))->format('Y-m-d\\TH:i:s.vP');

      }

} Maintaining such a set of Formatters in Userland seems to be much easier than adding that in the PHP-Src. It would also make the formatter available to ALL PHP-Versions that have a DateTimeImmutable object and not only PHP8.6/8.7 onwards. From my side I would by now not add any more constants to the PHP-Source as they will not be available to earler versions. And I say that as the person that added the _EXTENDED constants... By now I would create a formatter in userland and promote that for usage as it's a small library that can be used by *any* PHP-Version that needs it. My 0.02€ Cheers Andreas --

                                                                ,,,
                                                               (o o)

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| Andreas Heigl                                                       |
| mailto:andreas@heigl.org                  N 50°22'59.5" E 08°23'58" |
| https://andreas.heigl.org                                           |

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