RFC 867 defines the Daytime protocol.

… daytime service service is defined as a datagram based application on UDP. A server listens for UDP datagrams on UDP port 13. When a datagram is received, an answering datagram is sent containing the current date and time as a ASCII character string (the data in the received datagram is ignored).

See also TCP/13 for the TCP version of this protocol.

IANA Data

Name: daytime

Description: Daytime