IntlDateFormatter::format() displays weekday and week number incorrectly.

[Wiimm]

Description

Class IntlDateFormatter displays weekday and week number incorrectly. The example date is 2021-01-02, which belongs to the last week of 2020. In addition, the number of the day of the week is displayed incorrectly.

<?PHP

# tested with PHP 7.2.5 + 7.4.6 + 8.3.6 + 8.3.7
# output always:
#> SFT: 2021-01-02 12:00:00 / wday: 6=Sat / week: 2020-53
#> IDF: 2021-01-02 12:00:00 / wday: 7=Sat / week: 2021-01
# ERROR:                            ^             ^^^^ ^^

$locale   = 'en_US.UTF-8';
$timezone = 'UTC';
$time     = strtotime('2021-01-02 12:00');

setlocale(LC_TIME,$locale);
date_default_timezone_set($timezone);

echo strftime('SFT: %F %T / wday: %u=%a / week: %G-%V%n',$time);

$idf = new IntlDateFormatter(
		$locale,
		\IntlDateFormatter::FULL,
		\IntlDateFormatter::FULL,
		$timezone,
		\IntlDateFormatter::GREGORIAN,
		"yyyy-MM-dd HH:mm:ss '/ wday:' e=E '/ week:' Y-ww\n" );
echo 'IDF: ',$idf->format($time);
?>

Resulted in this output:

SFT: 2021-01-02 12:00:00 / wday: 6=Sat / week: 2020-53
IDF: 2021-01-02 12:00:00 / wday: 7=Sat / week: 2021-01
                                 ^             ^^^^ ^^

But I expected this output instead:

SFT: 2021-01-02 12:00:00 / wday: 6=Sat / week: 2020-53
IDF: 2021-01-02 12:00:00 / wday: 6=Sat / week: 2020-53
                                 ^             ^^^^ ^^

PHP Version

PHP 7.2.5 .. 8.3.7

Operating System

openSUSE Leap + Tumbleweed, Debian 11

[damianwadley]

The issue is the definition of the IntlDateFormatter::GREGORIAN calendar: it has the first week of the ISO year using a minimum of 1 day, instead of the usual 4 days. It also has Sunday as the first day of the week instead of Monday. So that's why it says Saturday is 7 and the week is 2021-01.

If I create an IntlCalendar with Monday as the first day of the week and a minimum of 4 days in the first week, like the 8601 standard typically uses, and pass that to IntlDateFormatter, I get 6=Sat and 2020-53.

"Typically". We've seen this problem before.

[Wiimm]

If I add

    $ic = IntlCalendar::createInstance('UTC');
    $ic->setMinimalDaysInFirstWeek(4);
    $ic->setFirstDayOfWeek(IntlCalendar::DOW_MONDAY);

and use the new object instead of IntlDateFormatter::GREGORIAN, then it works like expected.

@damianwadley Thank you for the info.