Binary package hint: file anon@x-X:~$ echo "1/2 Tue" >> file && file file file: Jan 22 14:32:44 MET 1991\011Erlang JAM file - version 4.2 anon@x-X:~$ file --version file-4.21 magic file from /etc/magic:/usr/share/file/magic If you look at the magic file: 4 string Tue Jan 22 14:32:44 MET 1991 Erlang JAM file - version 4.2 So at the fourth byte of the file, it's looking for Tue Jan 22 14:32:44 MET 1