Es scheint zu klappen, vielen herzlichen Dank!
Nur um sicher zu gehen, wäre dies dann der richtige PHP-Code für die Erinnerungsmails?
$base = time();
if ($base > strtotime('19:30:00')) {
$base = strtotime('next day 00:00:00');
}
mailSchedule(false, 38, strtotime('+0 day 19:30:00', $base));
mailSchedule(false, 40, strtotime('+1 day 19:30:00', $base));
mailSchedule(false, 42, strtotime('+2 day 19:30:00', $base));
mailSchedule(false, 44, strtotime('+3 day 19:30:00', $base));
mailSchedule(false, 46, strtotime('+4 day 19:30:00', $base));
mailSchedule(false, 48, strtotime('+5 day 19:30:00', $base));
mailSchedule(false, 50, strtotime('+6 day 19:30:00', $base));
mailSchedule(false, 39, strtotime('+0 day 19:30:00', $base)+3600*13, ['status' => 'incomplete']);
mailSchedule(false, 41, strtotime('+1 day 19:30:00', $base)+3600*13, ['status' => 'incomplete']);
mailSchedule(false, 43, strtotime('+2 day 19:30:00', $base)+3600*13, ['status' => 'incomplete']);
mailSchedule(false, 45, strtotime('+3 day 19:30:00', $base)+3600*13, ['status' => 'incomplete']);
mailSchedule(false, 47, strtotime('+4 day 19:30:00', $base)+3600*13, ['status' => 'incomplete']);
mailSchedule(false, 49, strtotime('+5 day 19:30:00', $base)+3600*13, ['status' => 'incomplete']);
mailSchedule(false, 51, strtotime('+6 day 19:30:00', $base)+3600*13, ['status' => 'incomplete']);