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Network Time Protocol
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== Timestamps == The 64-bit [[fixed-point arithmetic|binary fixed-point]] timestamps used by NTP consist of a 32-bit part for seconds and a 32-bit part for fractional second, giving a time scale that [[Integer overflow|rolls over]] every 2<sup>32</sup> seconds (136 years) and a theoretical resolution of 2<sup>β32</sup> seconds (233 picoseconds). NTP uses an [[epoch]] of January 1, 1900. Therefore, the first rollover occurs on February 7, 2036.<ref name="mvUS1">{{cite web |url=https://www.eecis.udel.edu/~mills/y2k.html |author=David L. Mills |title=The NTP Era and Era Numbering |date=12 May 2012 |access-date=24 September 2016 |archive-url=https://web.archive.org/web/20161026011515/https://www.eecis.udel.edu/~mills/y2k.html |archive-date=26 October 2016 |url-status=live}}</ref><ref name="StevensFenner2004">{{cite book |author1=W. Richard Stevens |author2=Bill Fenner |author3=Andrew M. Rudoff |title=UNIX Network Programming |url=https://books.google.com/books?id=ptSC4LpwGA0C&pg=PA582 |year=2004 |publisher=Addison-Wesley Professional |isbn=978-0-13-141155-5 |pages=582β |access-date=2016-10-16 |archive-url=https://web.archive.org/web/20190330170620/https://books.google.com/books?id=ptSC4LpwGA0C&pg=PA582 |archive-date=2019-03-30 |url-status=live}}</ref> NTPv4 introduces a 128-bit date format: 64 bits for the second and 64 bits for the fractional-second. The most-significant 32 bits of this format is the ''Era Number'' which resolves rollover ambiguity in most cases.<ref name="bzRE9">{{cite web |title=A look at the Year 2036/2038 problems and time proofness in various systems |date=14 March 2017 |url=http://www.lieberbiber.de/2017/03/14/a-look-at-the-year-20362038-problems-and-time-proofness-in-various-systems/ |access-date=2018-07-20 |archive-url=https://web.archive.org/web/20180721014309/http://www.lieberbiber.de/2017/03/14/a-look-at-the-year-20362038-problems-and-time-proofness-in-various-systems/ |archive-date=2018-07-21 |url-status=live}}</ref> According to Mills, "The 64-bit value for the fraction is enough to resolve the amount of time it takes a photon to pass an electron at the speed of light. The 64-bit second value is enough to provide unambiguous time representation until the universe goes dim."<ref name="FIILt">[[University of Delaware]] Digital Systems Seminar presentation by David Mills, 2006-04-26</ref>{{efn|2<sup>β64</sup> seconds is about [[1 E-21 s|54 zeptoseconds]] (light would travel 16.26 picometers, or approximately 0.31 Γ [[Bohr radius]]), and 2<sup>64</sup> seconds is about [[1 E19 s and more|585 billion years]].}}
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