diff --git a/backend/server.js b/backend/server.js
index e5c6bd8..33b3715 100644
--- a/backend/server.js
+++ b/backend/server.js
@@ -945,6 +945,18 @@ function normalizeDeadline(value) {
   return null;
 }
 
+function isDeadlineExpired(value, now = new Date()) {
+  const normalized = normalizeDeadline(value);
+  if (!normalized) {
+    return false;
+  }
+  const deadline = new Date(normalized);
+  if (Number.isNaN(deadline.getTime())) {
+    return false;
+  }
+  return deadline.getTime() <= now.getTime();
+}
+
 function getProfileName(profileNumber) {
   return DEFAULT_PROFILE_NAMES[profileNumber] || `Profil ${profileNumber}`;
 }
@@ -6506,6 +6518,9 @@ app.post('/api/posts', (req, res) => {
       if (!normalizedDeadline) {
         return res.status(400).json({ error: 'deadline_at must be a valid date string' });
       }
+      if (isDeadlineExpired(normalizedDeadline)) {
+        return res.status(400).json({ error: 'Deadline ist bereits abgelaufen' });
+      }
     }
 
     const creatorDisplayName = normalizeCreatorName(created_by_name);
@@ -6625,6 +6640,9 @@ app.put('/api/posts/:postId', (req, res) => {
         if (!normalizedDeadline) {
           return res.status(400).json({ error: 'deadline_at must be a valid date string' });
         }
+        if (isDeadlineExpired(normalizedDeadline)) {
+          return res.status(400).json({ error: 'Deadline ist bereits abgelaufen' });
+        }
       }
       updates.push('deadline_at = ?');
       params.push(normalizedDeadline);