diff --git a/sphinx/environment.py b/sphinx/environment.py
index d74b323da..88e9f99d7 100644
--- a/sphinx/environment.py
+++ b/sphinx/environment.py
@@ -616,8 +616,8 @@ class BuildEnvironment:
         self.note_citations_from(docname, doctree)
         self.build_toc_from(docname, doctree)
 
-        # store file modification time, for outdated files detection
-        self.all_docs[docname] = path.getmtime(src_path)
+        # store time of build, for outdated files detection
+        self.all_docs[docname] = time.time()
 
         if app:
             app.emit('doctree-read', doctree)