Simple Network Time Protocol

The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. NTP uses UDP port 123 as its transport layer. It is designed particularly to resist the effects of variable latency (jitter).

NTP is one of the oldest Internet protocols still in use (since before 1985). NTP was originally designed by Dave Mills of the University of Delaware, who still maintains it, along with a team of volunteers.

NTP is not related to the much simpler DAYTIME (RFC 867) and TIME (RFC 868) protocols.

NTP uses Marzullo's algorithm with the UTC time scale, including support for features such as leap seconds. NTPv4 can usually maintain time to within 10 milliseconds (1/100 s) over the public Internet, and can achieve accuracies of 200 microseconds (1/5000 s) or better in local area networks under ideal conditions.

A less complex form of NTP that does not require storing information about previous communications is known as the Simple Network Time Protocol or SNTP. It is used in some embedded devices and in applications where high accuracy timing is not required. See RFC 1361, RFC 1769, RFC 2030, and RFC 4330.

Year 2036 problem

The SNTP protocol uses a 64-bit unsigned number, consisting from two 32-bit parts. First part is number of seconds from 0:00:00 01 Jan 1900 GMT, which overflows and turns back to 0 on 6:28:16 20 Feb 2036 GMT. Second part contains fractions of seconds in picosecond accuracy.

To fix the incorrect time representation, Advanced Time Synchronizer from version 6 and later uses the high bit of the 32-bit unsigned number to separate time periods.

When the high bit is set, the 32-bit unsigned number represents the period from 3:14:08 20 Jan 1968 GMT to 6:28:15 20 Feb 2036 GMT.

When the high bit is not set, the 32-bit unsigned number represents the period from 6:28:16 20 Feb 2036 GMT to 9:42:23 26 Feb 2104 GMT.

Therefore, the representation range is from 3:14:08 20 Jan 1968 GMT to 9:42:23 26 Feb 2104 GMT.

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