diff --git a/backend/app.py b/backend/app.py
index 3525c6b..fcef01c 100644
--- a/backend/app.py
+++ b/backend/app.py
@@ -313,6 +313,11 @@ def get_all_data():
     if messages_missing_parent > 0:
         logger.warning(f"Found {messages_missing_parent} messages that could not be linked to a conversation")
     
+    # Log warning if any messages ended up without a Timestamp after enrichment
+    msg_no_timestamp = sum(1 for m in enriched_messages if not m.get("Timestamp"))
+    if msg_no_timestamp:
+        logger.warning(f"Messages missing Timestamp after enrichment: {msg_no_timestamp} / {len(enriched_messages)}")
+
     # Log warning if messages are missing Assistant_ID
     msg_has_assistant_id = sum(1 for msg in enriched_messages if msg.get("Assistant_ID"))
     msg_no_assistant_id = sum(1 for msg in enriched_messages if not msg.get("Assistant_ID"))
diff --git a/frontend/src/components/Dashboard.jsx b/frontend/src/components/Dashboard.jsx
index 71bb35d..7c4be5c 100644
--- a/frontend/src/components/Dashboard.jsx
+++ b/frontend/src/components/Dashboard.jsx
@@ -99,22 +99,26 @@ const Dashboard = ({ conversations, messages }) => {
       
       let matches = true;
       
-      // Date range filter - only apply if both start and end dates are set
-      if (filters.dateRange.start && filters.dateRange.end && conv.StartTime) {
-        try {
-          const convDate = parseISO(conv.StartTime);
-          if (isValid(convDate)) {
-            matches = matches && isWithinInterval(convDate, { 
-              start: filters.dateRange.start, 
-              end: filters.dateRange.end 
-            });
-          } else {
-            console.warn("Invalid date in conversation:", conv.StartTime);
+      // Date range filter - fail-closed: missing/unparseable StartTime is excluded when a range is active
+      if (filters.dateRange.start && filters.dateRange.end) {
+        if (!conv.StartTime) {
+          matches = false;
+        } else {
+          try {
+            const convDate = parseISO(conv.StartTime);
+            if (!isValid(convDate)) {
+              console.warn("Invalid date in conversation:", conv.StartTime);
+              matches = false;
+            } else {
+              matches = matches && isWithinInterval(convDate, {
+                start: filters.dateRange.start,
+                end: filters.dateRange.end,
+              });
+            }
+          } catch (e) {
+            console.error("Error parsing conversation date:", e);
             matches = false;
           }
-        } catch (e) {
-          console.error("Error parsing conversation date:", e);
-          matches = false;
         }
       }
       
@@ -157,22 +161,26 @@ const Dashboard = ({ conversations, messages }) => {
       
       let matches = true;
       
-      // Date range filter - only apply if both start and end dates are set
-      if (filters.dateRange.start && filters.dateRange.end && msg.Timestamp) {
-        try {
-          const msgDate = parseISO(msg.Timestamp);
-          if (isValid(msgDate)) {
-            matches = matches && isWithinInterval(msgDate, { 
-              start: filters.dateRange.start, 
-              end: filters.dateRange.end 
-            });
-          } else {
-            console.warn("Invalid date in message:", msg.Timestamp);
+      // Date range filter - fail-closed: missing/unparseable Timestamp is excluded when a range is active
+      if (filters.dateRange.start && filters.dateRange.end) {
+        if (!msg.Timestamp) {
+          matches = false;
+        } else {
+          try {
+            const msgDate = parseISO(msg.Timestamp);
+            if (!isValid(msgDate)) {
+              console.warn("Invalid date in message:", msg.Timestamp);
+              matches = false;
+            } else {
+              matches = matches && isWithinInterval(msgDate, {
+                start: filters.dateRange.start,
+                end: filters.dateRange.end,
+              });
+            }
+          } catch (e) {
+            console.error("Error parsing message date:", e);
             matches = false;
           }
-        } catch (e) {
-          console.error("Error parsing message date:", e);
-          matches = false;
         }
       }