Measuring system clock (time) of a remote server

Question

What are the network protocols that can be used to measure the system clock (time) of a remote server? So far I have:

• Query with NTP
• TCP Timestamps
• HTTP headers

Are there any other?

Answer 1

There's always the Daytime Protocol

From the RFC:

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
quote.

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).