Backport PR #8111: Fix error when log_datefmt is changed

Fixes  #8110

IPython/config/application.py

@@ -159,7 +159,7 b' class Application(SingletonConfigurable):'
         help="The date format used by logging formatters for %(asctime)s"
     )
     def _log_datefmt_changed(self, name, old, new):
-        self._log_format_changed()
+        self._log_format_changed('log_format', self.log_format, self.log_format)
     
     log_format = Unicode("[%(name)s]%(highlevel)s %(message)s", config=True,
         help="The Logging format template",

IPython/config/tests/test_application.py

@@ -80,6 +80,7 b' class TestApplication(TestCase):'
         # trigger reconstruction of the log formatter
         app.log.handlers = [handler]
         app.log_format = "%(message)s"
+        app.log_datefmt = "%Y-%m-%d %H:%M"
         app.log.info("hello")
         nt.assert_in("hello", stream.getvalue())