test-date - PHP8

<?php

$twentiethmonday = new DateTime(date('Y-01-01'));
$twentiethmonday->modify('+20 Monday');
$datestr = $twentiethmonday->format('Ymd\T090000');
$timezonestr = 'US/Eastern';
$timezone = new DateTimeZone($timezonestr);
$startdatetime = DateTime::createFromFormat('Ymd\THis', $datestr, $timezone);

echo $startdatetime->format('Y-m-d H:i:s') . "\n\n";

$startdate = new DateTime($startdatetime->format('Y-m-d'));
$offset = $startdatetime->diff($startdate, true);
$interval = new DateInterval('P1Y');

$startdatetime->modify('January 1');
$startdatetime->add($interval);
$startdatetime->modify("+20 Monday");
$startdatetime->add($offset);

echo $startdatetime->format('Y-m-d H:i:s') . "\n";