The coolest implementation of the Daytime protocol are the programs that syncronize your clock with any number of atomic clock servers around the world. Now here's the text of the original RFC.
Network Working Group J. Postel
Request for Comments: 867 ISI
This RFC specifies a standard for the ARPA Internet community. Hosts on
the ARPA Internet that choose to implement a Daytime Protocol are
expected to adopt and implement this standard.
A useful debugging and measurement tool is a daytime service. A daytime
service simply sends a the current date and time as a character string
without regard to the input.
TCP Based Daytime Service
One 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
UDP Based Daytime Service
Another 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).
There is no specific syntax for the daytime. It is recommended that
it be limited to the ASCII printing characters, space, carriage
return, and line feed. The daytime should be just one line.
One popular syntax is:
Weekday, Month Day, Year Time-Zone
Tuesday, February 22, 1982 17:37:43-PST
RFC 867 May 1983
Another popular syntax is that used in SMTP:
dd mmm yy hh:mm:ss zzz
02 FEB 82 07:59:01 PST
NOTE: For machine useful time use the Time Protocol (RFC-868).