diff -r df3660cc60f5 -r b4b6ff83ed9c mercurial/ui.py
--- a/mercurial/ui.py	Wed Jul 01 14:28:12 2020 -0400
+++ b/mercurial/ui.py	Thu Jul 02 19:54:44 2020 +0200
@@ -1229,7 +1229,8 @@
         }
         if timestamp:
             args = (
-                b'[%s] ' % bytes(datetime.datetime.now().isoformat(), 'ASCII'),
+                b'[%s] '
+                % pycompat.bytestr(datetime.datetime.now().isoformat()),
             ) + args
         _writemsgwith(self._write, dest, *args, **opts)
         if timestamp: