diff --git a/index.php b/index.php
index eb21bfb..6a99346 100644
--- a/index.php
+++ b/index.php
@@ -1,4 +1,5 @@
 <?php
+locale_set_default('de_DE');
 $config = require_once './config.php';
 $START_DATE = $config['start_date'];
 $EVENTS = $config['events'];
@@ -45,7 +46,7 @@ function print_month($start) {
   <caption><h3><?= $month_name ?></caption>
   <tr><?php foreach ($DAYS_OF_WEEK as $day) { echo "<th>$day</th>"; }?></tr>
 <?php
-  $date = DateTime::createFromInterface($week_start);
+  $date = DateTime::createFromImmutable($week_start);
   while ($date->diff($month_end)->invert == 0) {
     echo "<tr>";
     for ($n = 0; $n < 7; ++$n) {