Datetimes are hard to get right...

datetime_wtf.php

<?php

/*
 * The timezone in these datetimes look the same but they are in fact represent different UTC offsets.
 * The 1st one maps to UTC+01 (BST) while the 2nd one maps to a UTC+00 (GMT).
 * This is because daylight saving time (DST) aka. clocks going forward/backward happen on different days in different years.
 */
$now     = new DateTime('2018-03-26T08:00:00', new DateTimeZone('Europe/London'));
$inAYear = new DateTime('2019-03-26T08:00:00', new DateTimeZone('Europe/London'));

echo $now->format(DATE_ATOM) . PHP_EOL;     // 2018-03-26T08:00:00+01:00
echo $inAYear->format(DATE_ATOM) . PHP_EOL; // 2019-03-26T08:00:00+00:00
echo $now->getTimezone()->getOffset($now) . PHP_EOL;         // 3600
echo $inAYear->getTimezone()->getOffset($inAYear) . PHP_EOL; // 0

/*
 * If we set both timezone values to an absolute UTC+01 offset then the 1st one generates a valid
 * Europe/London BST datetime, however, the second one is not a valid Europe/London datetime
 * as on that datetime Europe/London maps to UTC+00 (GMT).
 */
$now     = new DateTime('2018-03-26T08:00:00+01:00');
$inAYear = new DateTime('2019-03-26T08:00:00+01:00'); // not valid Europe/London

echo $now->format(DATE_ATOM) . PHP_EOL;     // 2018-03-26T08:00:00+01:00
echo $inAYear->format(DATE_ATOM) . PHP_EOL; // 2019-03-26T08:00:00+01:00
echo $now->getTimezone()->getOffset($now) . PHP_EOL;         // 3600
echo $inAYear->getTimezone()->getOffset($inAYear) . PHP_EOL; // 3600