Welcome to HTTP Time protocol
The HTTP Time Protocol (HTP) is used to synchronize a computer's time with web servers as reference time source. The htpdate utility retrieves the date and time from remote web servers via the network. Htpdate works through proxy servers as it behaves similar to a browser. Accuracy of htpdate will be usually within 0.5 seconds (better with multiple servers).
Time synchronization protocols
Many implementations exist for time synchronisation between computers over a network. Below you will find a short list of popular network protocols which are in use today:
daytime (RFC 867) protocol sends a human readable string when connecting to UDP or TCP port 13
> telnet time.nist.gov 13
Trying 2610:20:6f15:15::27...
Connected to time.nist.gov.
Escape character is '^]'.
56941 14-10-11 14:42:10 23 0 0 252.1 UTC(NIST) *
Connection closed by foreign host.time (RFC 868) protocol is similar to the daytime protocol described above and used by rdate. The format of the time given by the time server is in "computer readable" format.
timed daemon uses ICMP messages for time synchronization. Most routers and hosts don't support ICMP timestamp requests.
ntp (RFC 1305, RFC 2030, www.ntp.org, chrony) is the most popular time synchronization protocol in use nowadays. NTP has good accuracy (milliseconds across Internet and microseconds or less in LAN environments) with only a small amount of network traffic.
The time synchronization challenge
Why would anybody need another time protocol, when ntp is already great? A simple answer is "you don't", unless you don't have access to a ntp server, because you are behind a firewall and/or proxy server.
In order for ntp to work, you need to have access to outgoing and incoming packets from UDP port 123. In most corporate environments this is not the case and you must consider yourself lucky if you have Internet access via an HTTP proxy server. As long as HTTP connectivity is available, you can use HTP/htpdate.