I was using `IntlGregorianCalendar` because it offered a nice way to get the week number of the year as an integer. This differs from `DateTime`; `DateTime` gives you the week number of the year as an string.
<?php
$dateTime = new DateTime('21-09-2020 09:00:00');
echo $dateTime->format("W"); // string '39'
$intlCalendar = IntlCalendar::fromDateTime ('21-09-2020 09:00:00');
echo $intlCalendar->get(IntlCalendar::FIELD_WEEK_OF_YEAR); // integer 39