Daytime Protocol
Connect to port 13 and get the date and time as a line of human-readable text — in a format nobody ever agreed on.
Jon Postel defined it in RFC 867 in 1983. The server sends the current time as ASCII and closes the connection; over UDP an empty datagram is enough to get a reply. The spec says outright that there is no specific syntax, offering only a couple of suggestions. Every Unix flavour duly invented its own, and any program that wanted to parse the answer had to guess.
That was fine for a human wondering what a remote machine thought the time was, and useless for setting clocks. That job went to the machine-readable Time Protocol and then to NTP. NIST has long answered on port 13 with its own well-documented line format — possibly the only daytime server whose output you could actually rely on.