Always say why something is out of date

sphinx/environment/__init__.py

@@ -502,10 +502,35 @@ def get_outdated_files(self, config_changed: bool) -> tuple[set[str], set[str],
                         # this will do the right thing when dep is absolute too
                         deppath = path.join(self.srcdir, dep)
                         if not path.isfile(deppath):
+                            logger.debug(
+                                '[build target] changed %r '
+                                'missing dependency %r',
+                                docname,
+                                deppath)
                             changed.add(docname)
                             break
                         depmtime = _last_modified_time(deppath)
                         if depmtime > mtime:
+                            try:
+                                depmtime_dt = datetime.utcfromtimestamp(depmtime)
+                            except ValueError:  # e.g., year 53606865 is out of range
+                                depmtime_str = str(depmtime)
+                            else:
+                                depmtime_str = str(depmtime_dt)
+                            try:
+                                mtime_dt = datetime.utcfromtimestamp(mtime)
+                            except ValueError:
+                                mtime_str = str(mtime)
+                            else:
+                                mtime_str = str(mtime_dt)
+                            logger.debug(
+                                '[build target] outdated %r '
+                                'from dependency %r: %s -> %s',
+                                docname,
+                                deppath,
+                                mtime_str,
+                                depmtime_str,
+                            )
                             changed.add(docname)
                             break
                     except OSError: