diff --git a/mkdocs/commands/build.py b/mkdocs/commands/build.py
index 2c9e9793..2eec30aa 100644
--- a/mkdocs/commands/build.py
+++ b/mkdocs/commands/build.py
@@ -110,7 +110,9 @@ def _build_theme_template(
             log.debug(f"Gzipping template: {template_name}")
             gz_filename = f'{output_path}.gz'
             with open(gz_filename, 'wb') as f:
-                timestamp = utils.get_build_timestamp()
+                timestamp = utils.get_build_timestamp(
+                    pages=[f.page for f in files.documentation_pages() if f.page is not None]
+                )
                 with gzip.GzipFile(
                     fileobj=f, filename=gz_filename, mode='wb', mtime=timestamp
                 ) as gz_buf:
diff --git a/mkdocs/structure/pages.py b/mkdocs/structure/pages.py
index 84c1430f..f5bd041f 100644
--- a/mkdocs/structure/pages.py
+++ b/mkdocs/structure/pages.py
@@ -42,7 +42,7 @@ class Page(StructureItem):
         self.next_page = None
         self.active = False
 
-        self.update_date = get_build_date()
+        self.update_date: str = get_build_date()
 
         self._set_canonical_url(config.get('site_url', None))
         self._set_edit_url(
diff --git a/mkdocs/tests/build_tests.py b/mkdocs/tests/build_tests.py
index 95145298..ae1b474c 100644
--- a/mkdocs/tests/build_tests.py
+++ b/mkdocs/tests/build_tests.py
@@ -225,7 +225,7 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
     def test_build_theme_template(self, mock_build_template, mock_write_file):
         cfg = load_config()
         env = cfg.theme.get_env()
-        build._build_theme_template('main.html', env, mock.Mock(), cfg, mock.Mock())
+        build._build_theme_template('main.html', env, Files([]), cfg, mock.Mock())
         mock_write_file.assert_called_once()
         mock_build_template.assert_called_once()
 
@@ -238,7 +238,7 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
     ):
         cfg = load_config(site_dir=site_dir)
         env = cfg.theme.get_env()
-        build._build_theme_template('sitemap.xml', env, mock.Mock(), cfg, mock.Mock())
+        build._build_theme_template('sitemap.xml', env, Files([]), cfg, mock.Mock())
         mock_write_file.assert_called_once()
         mock_build_template.assert_called_once()
         mock_gzip_gzipfile.assert_called_once()
@@ -249,7 +249,7 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
         cfg = load_config()
         env = cfg.theme.get_env()
         with self.assertLogs('mkdocs') as cm:
-            build._build_theme_template('missing.html', env, mock.Mock(), cfg, mock.Mock())
+            build._build_theme_template('missing.html', env, Files([]), cfg, mock.Mock())
         self.assertEqual(
             '\n'.join(cm.output),
             "WARNING:mkdocs.commands.build:Template skipped: 'missing.html' not found in theme directories.",
@@ -263,7 +263,7 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
         cfg = load_config()
         env = cfg.theme.get_env()
         with self.assertLogs('mkdocs') as cm:
-            build._build_theme_template('main.html', env, mock.Mock(), cfg, mock.Mock())
+            build._build_theme_template('main.html', env, Files([]), cfg, mock.Mock())
         self.assertEqual(
             '\n'.join(cm.output),
             "INFO:mkdocs.commands.build:Template skipped: 'main.html' generated empty output.",
diff --git a/mkdocs/utils/__init__.py b/mkdocs/utils/__init__.py
index 1560dfa1..602e99fe 100644
--- a/mkdocs/utils/__init__.py
+++ b/mkdocs/utils/__init__.py
@@ -44,18 +44,19 @@ markdown_extensions = (
 )
 
 
-def get_build_timestamp() -> int:
+def get_build_timestamp(*, pages: Collection[Page] | None = None) -> int:
     """
-    Returns the number of seconds since the epoch.
+    Returns the number of seconds since the epoch for the latest updated page.
 
-    Support SOURCE_DATE_EPOCH environment variable for reproducible builds.
-    See https://reproducible-builds.org/specs/source-date-epoch/
+    In reality this is just today's date because that's how pages' update time is populated.
     """
-    source_date_epoch = os.environ.get('SOURCE_DATE_EPOCH')
-    if source_date_epoch is None:
-        return int(datetime.now(timezone.utc).timestamp())
-
-    return int(source_date_epoch)
+    if pages:
+        # Lexicographic comparison is OK for ISO date.
+        date_string = max(p.update_date for p in pages)
+        dt = datetime.fromisoformat(date_string)
+    else:
+        dt = get_build_datetime()
+    return int(dt.timestamp())
 
 
 def get_build_datetime() -> datetime: