The following patch does the trick, but I'm not sure, if that's the right way to fix it.
(Maybe "hour_meridian" should be renamed to "hour_in_view" or something like that?)
Regards,
Peter
--- a/src/view/phui/calendar/PHUICalendarDayView.php +++ b/src/view/phui/calendar/PHUICalendarDayView.php @@ -61,7 +61,7 @@ final class PHUICalendarDayView extends AphrontView { foreach ($hours as $hour) { $js_hours[] = array( 'hour' => $hour->format('G'), - 'hour_meridian' => $hour->format('g A'), + 'hour_meridian' => phabricator_time($hour->getTimestamp(), $viewer), ); }