Add ISO 8601 date format constant with millisecond precision

Hey Théo

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

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

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