And why? Because right now:

$dt = new DateTimeImmutable('2026-12-01 10:00:00');
$formatter = new IntlDateFormatter(
'en_US',
IntlDateFormatter::FULL,
IntlDateFormatter::FULL,
'America/Edmonton'
);
echo $formatter->format($dt);

results in

Tuesday, December 1, 2026 at 3:00:00 AM Mountain Standard Time

While it should be "4:00:00 AM Alberta Time" instead of "3:00:00 AM Mountain Standard Time"