RFC 867 defines the Daytime protocol.
... daytime service is defined as a connection based application on TCP. A server listens for TCP connections on TCP port 13. Once a connection is established the current date and time is sent out the connection as a ascii character string (and any data received is thrown away). The service closes the connection after sending the quote.
See also UDP/13 for the UDP version of this protocol.
IANA Data
Name: daytime
Description: Daytime