diff --git a/opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/IndexTimestamp.java b/opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/IndexTimestamp.java
index 305105277ae..0e25fc9fcc7 100644
--- a/opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/IndexTimestamp.java
+++ b/opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/IndexTimestamp.java
@@ -18,7 +18,7 @@
  */
 
  /*
- * Copyright (c) 2017, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2017, 2025, Oracle and/or its affiliates. All rights reserved.
  */
 package org.opengrok.indexer.configuration;
 
@@ -27,6 +27,9 @@
 import java.util.Date;
 import java.util.logging.Level;
 import java.util.logging.Logger;
+
+import org.jetbrains.annotations.Nullable;
+import org.jetbrains.annotations.VisibleForTesting;
 import org.opengrok.indexer.logger.LoggerFactory;
 
 public class IndexTimestamp {
@@ -34,15 +37,18 @@ public class IndexTimestamp {
 
     private static final Logger LOGGER = LoggerFactory.getLogger(IndexTimestamp.class);
 
+    @VisibleForTesting
+    public static final String TIMESTAMP_FILE_NAME = "timestamp";
+
     /**
      * Get the date of the last index update.
      *
-     * @return the time of the last index update.
+     * @return the time of the last index update or {@code null}.
      */
-    public Date getDateForLastIndexRun() {
+    public @Nullable Date getDateForLastIndexRun() {
         RuntimeEnvironment env = RuntimeEnvironment.getInstance();
         if (lastModified == null) {
-            File timestamp = new File(env.getDataRootFile(), "timestamp");
+            File timestamp = new File(env.getDataRootFile(), TIMESTAMP_FILE_NAME);
             if (timestamp.exists()) {
                 lastModified = new Date(timestamp.lastModified());
             }
diff --git a/opengrok-web/src/test/java/org/opengrok/web/PageConfigTest.java b/opengrok-web/src/test/java/org/opengrok/web/PageConfigTest.java
index e0dd781ac01..5f30fb2f9fa 100644
--- a/opengrok-web/src/test/java/org/opengrok/web/PageConfigTest.java
+++ b/opengrok-web/src/test/java/org/opengrok/web/PageConfigTest.java
@@ -49,11 +49,13 @@
 import org.junit.jupiter.api.condition.OS;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.opengrok.indexer.authorization.AuthControlFlag;
 import org.opengrok.indexer.authorization.AuthorizationFramework;
 import org.opengrok.indexer.authorization.AuthorizationPlugin;
 import org.opengrok.indexer.authorization.TestPlugin;
 import org.opengrok.indexer.condition.EnabledForRepository;
+import org.opengrok.indexer.configuration.IndexTimestamp;
 import org.opengrok.indexer.configuration.Project;
 import org.opengrok.indexer.configuration.RuntimeEnvironment;
 import org.opengrok.indexer.history.Annotation;
@@ -655,8 +657,9 @@ public String getPathInfo() {
         };
     }
 
-    @Test
-    void testIsNotModifiedEtag() {
+    @ParameterizedTest
+    @ValueSource(booleans = {true, false})
+    void testIsNotModifiedEtag(boolean createTimestamp) throws IOException {
         HttpServletRequest req = new DummyHttpServletRequest() {
             @Override
             public String getHeader(String name) {
@@ -672,6 +675,16 @@ public String getPathInfo() {
             }
         };
 
+        // The ETag value depends on the timestamp file.
+        RuntimeEnvironment env = RuntimeEnvironment.getInstance();
+        env.refreshDateForLastIndexRun();
+        Path timestampPath = Path.of(env.getDataRootPath(), IndexTimestamp.TIMESTAMP_FILE_NAME);
+        Files.deleteIfExists(timestampPath);
+        if (createTimestamp) {
+            Files.createFile(timestampPath);
+            assertTrue(timestampPath.toFile().exists());
+        }
+
         PageConfig cfg = PageConfig.get(req);
         HttpServletResponse resp = mock(HttpServletResponse.class);
         assertFalse(cfg.isNotModified(req, resp));