Add a UTC-normalizing DateTime method for ISO 8601 output

PHP provides predefined date format constants such as DATE_ISO8601,
DATE_ATOM, DATE_RFC3339, and DATE_RFC3339_EXTENDED. There is no predefined
format for the UTC timestamp commonly produced by JavaScript:

1970-01-01T00:00:00.000Z

For example:

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

Proposal

Add a predefined format for ISO 8601 timestamps with millisecond precision.

One possible name is:

DATE_ISO8601_MILLISECONDS_UTC
DateTimeInterface::ISO8601_MILLISECONDS_UTC

with the format string:

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

The name and API shape are open for discussion.

An alternative is an offset-preserving format:

DATE_ISO8601_MILLISECONDS
DateTimeInterface::ISO8601_MILLISECONDS

using:

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

Example

$date = new DateTimeImmutable('@0');

echo $date->setTimezone(new DateTimeZone('UTC'))
    ->format(DATE_ISO8601_MILLISECONDS_UTC);

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

As with the existing date format constants, formatting does not change the
timezone of the DateTime object. The caller must convert it to UTC before
using the _UTC format.

Existing constant

DATE_RFC3339_EXTENDED already includes milliseconds, but produces an offset:

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

This is semantically equivalent to Z for UTC, but is not byte-for-byte
compatible with JavaScript's Date.prototype.toISOString().

Related discussion

PHP issue #14593 discusses the
interpretation of the Z suffix when parsing DateTime values. That issue is
about parsing and timezone representation; this proposal is about predefined
formatting constants in ext/date.

Questions

• Is a millisecond-precision constant useful in addition to
  DATE_RFC3339_EXTENDED?
• Should the format preserve the object's offset, or provide the Z form for
  UTC-normalized values?
• What name and API shape should be used?
• Would both forms introduce unnecessary overlap with the existing constants?