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Epoch (computing)
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<h2 id="resolution-and-representation">Resolution and representation</h2> <p>Software timekeeping systems vary widely in the <a href="/facts/Temporal_resolution/9wp61G97">resolution</a> of time measurement; some systems may use time units as large as a day, while others may use <a href="/facts/Nanosecond/gARLTp4c">nanoseconds</a>. For example, for an epoch date of midnight UTC (00:00) on 1 January 1900, and a time unit of a second, the time of the midnight (24:00) between 1 January 1900 and 2 January 1900 is represented by the number 86400, the number of seconds in one day. When times prior to the epoch need to be represented, it is common to use the same system, but with negative numbers. </p><p>Such representation of time is mainly for internal use. On systems where date and time are important in the human sense, software will almost always convert this internal number into a date and time representing a human calendar. </p> <h2 id="problems">Problems</h2> <p>Computers do not generally store arbitrarily large numbers. Instead, each number stored by a computer is allotted a fixed amount of space. Therefore, when the number of time units that have elapsed since a system's epoch exceeds the largest number that can fit in the space allotted to the time representation, the time representation <a href="/facts/Arithmetic_overflow/sVNnSolT">overflows</a>, and problems can occur. While a system's behavior after overflow occurs is not necessarily predictable, in most systems the number representing the time will reset to zero, and the computer system will think that the current time is the epoch time again. </p><p>Most famously, older systems that counted time as the number of years elapsed since the epoch of 1 January 1900 and which only allotted enough space to store the numbers 0 through 99, experienced the <a href="/facts/Year_2000_problem/GQwkI92d">Year 2000 problem</a>. These systems (if not corrected beforehand) would interpret the date 1 January 2000 as 1 January 1900, leading to unpredictable errors at the beginning of the year 2000. </p><p>Even systems that allocate more storage to the time representation are not immune from this kind of error. Many <a href="/facts/Unix-like_operating_system/YAKrAxzd">Unix-like operating systems</a> which keep time as seconds elapsed from the epoch date of 1 January 1970, and allot timekeeping enough storage to store numbers as large as 2 147 483 647 will experience an overflow problem on 19 January 2038. This is known as the <a href="/facts/Year_2038_problem/dsvSnTgh">Year 2038 problem</a>. </p><p>Other more subtle timekeeping problems exist in computing, such as accounting for <a href="/facts/Leap_second/HQQ0X0hn">leap seconds</a>, which are not observed with any predictability or regularity. Additionally, applications that need to represent historical dates and times (for example, representing <a href="/facts/Old_Style_and_New_Style_dates/kb3020Wy">a date prior to the switch</a> from the <a href="/facts/Julian_calendar/EQBtA1ML">Julian calendar</a> to the <a href="/facts/Gregorian_calendar/axJfTXV1">Gregorian calendar</a>) must use specialized timekeeping <a href="/facts/Library_(computing)/xRzL3P7q">libraries</a>. </p><p>Finally, some software must maintain compatibility with older software that does not keep time in strict accordance with traditional timekeeping systems. For example, <a href="/facts/Microsoft_Excel/Ezal6G75">Microsoft Excel</a> observes the fictional date of <a href="/facts/29_February_1900/06vgfHzC">29 February 1900</a> in order to maintain <a href="/facts/Bug_compatibility/59rukF9g">bug compatibility</a> with older versions of <a href="/facts/Lotus_1-2-3/tnkvSG3M">Lotus 1-2-3</a>.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> Lotus 1-2-3 observed the date due to an error; by the time the error was discovered, it was too late to fix it—"a change now would disrupt formulas which were written to accommodate this anomaly".<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p> <h2 id="in-satellite-based-time-systems">In satellite-based time systems</h2> <p>There are at least six <a href="/facts/Satellite_navigation/lVgPlF5v">satellite navigation</a> systems, all of which function by transmitting <a href="/facts/Time_signal/pL6EnjJL">time signals</a>. Of the only two satellite systems with global coverage, <a href="/facts/GPS/DKVLnzF3">GPS</a> calculates its time signal from an epoch, whereas <a href="/facts/GLONASS/sLh1vQ6p">GLONASS</a> calculates time as an offset from <a href="/facts/UTC/2EnjfSyu">UTC</a>, with the UTC input adjusted for <a href="/facts/Leap_seconds/HQQ0X0hn">leap seconds</a>. Of the only two other systems aiming for global coverage, <a href="/facts/Galileo_(satellite_navigation)/sPyWl594">Galileo</a> calculates from an epoch and <a href="/facts/BeiDou/eW9Ex0WP">BeiDou</a> calculates from UTC without adjustment for leap seconds.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>[<i>needs update?</i>] GPS also transmits the offset between UTC time and GPS time and must update this offset every time there is a <a href="/facts/Leap_second/HQQ0X0hn">leap second</a>, requiring GPS-receiving devices to handle the update correctly. In contrast, leap seconds are transparent to GLONASS users. The complexities of calculating UTC from an epoch are explained by the <a href="/facts/European_Space_Agency/L8YRNV0C">European Space Agency</a> in Galileo documentation under "Equations to correct system timescale to reference timescale".<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> <h2 id="notable-epoch-dates-in-computing">Notable epoch dates in computing</h2> <p>The following table lists epoch dates used by popular software and other computer-related systems. The time in these systems is stored as the quantity of a particular time unit (days, seconds, nanoseconds, etc.) that has elapsed since a stated time (usually midnight UTC at the beginning of the given date). </p> <table><tbody><tr><th>Epoch date</th><th>Notable uses</th><th>Rationale for selection</th></tr><tr><td><a href="/facts/0_January/DoFhbkA0">0 January</a> <a href="/facts/Year_zero/Dur6Tb4o">1 BC</a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a></td><td><a href="/facts/MATLAB/qPjLISCk">MATLAB</a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a></td><td>"Year 0" in <a href="/facts/ISO_8601/4N8kykG9">ISO 8601</a></td></tr><tr><td>1 January AD 1<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a></td><td><a href="/facts/.NET/PDMQdfFu">.NET</a>,<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> <a href="/facts/Go_(programming_language)/oWlsmAFJ">Go</a>,<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> <a href="/facts/REXX/hAzh6FeR">REXX</a>,<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> <a href="/facts/Rata_Die/6fhRbdIA">Rata Die</a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a></td><td><a href="/facts/Common_Era/e5DpqUoe">Common Era</a>, <a href="/facts/ISO_2014/pdWWYXLf">ISO 2014</a>,<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> RFC 3339<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a></td></tr><tr><td>14 October 1582</td><td><a href="/facts/SPSS/6PGBVUXF">SPSS</a>,<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> IBM z/OS Language Environment,<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> <a href="/facts/IBM_AIX/cRWERua2">IBM AIX</a> <a href="/facts/COBOL/wVnhqs0k">COBOL</a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a></td><td>Same as below, but with <a href="/facts/One-based_indexing/gHJJ3XPw">one-based indexing</a></td></tr><tr><td>15 October 1582</td><td><a href="/facts/Universally_unique_identifier/SUtkYJPe">UUID version 1</a></td><td>The date of the <a href="/facts/Gregorian_calendar/axJfTXV1">Gregorian reform</a> to the Christian calendar.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a></td></tr><tr><td>1 January 1601</td><td><a href="/facts/NTFS/dryNM3BK">NTFS</a>, <a href="/facts/COBOL/wVnhqs0k">COBOL</a>,<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> <a href="/facts/Win32/RRw4RtTu">Win32</a>/<a href="/facts/Win64/RRw4RtTu">Win64</a> (NT time epoch)<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a><a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a></td><td>1601 was the first year of the 400-year Gregorian calendar cycle at the time Windows NT was made.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a></td></tr><tr><td>31 December 1840</td><td><a href="/facts/MUMPS/06E3hkoE">MUMPS</a> programming language</td><td>1841 was a non-leap year several years before the birth year of the oldest living US citizen when the language was designed.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a></td></tr><tr><td>17 November 1858</td><td><a href="/facts/OpenVMS/rAnHAgpD">VMS</a>, <a href="/facts/United_States_Naval_Observatory/81pxwM2R">United States Naval Observatory</a>, <a href="/facts/DVB/jzgt5Q48">DVB</a> SI 16-bit day stamps, other <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>-related computations<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a></td><td>17 November 1858, 00:00:00 UT is the zero of the <a href="/facts/Modified_Julian_Day/mLwtKoYQ">Modified Julian Day</a> (MJD) equivalent to <a href="/facts/Julian_day/mLwtKoYQ">Julian day</a> 2400000.5<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a></td></tr><tr><td>10 May 1869</td><td>PHAMIS date, used in <a href="/facts/IDX_Systems/crfyxUwK">Lastword/Carecast/Centricity Enterprise</a> EHR running on <a href="/facts/NonStop_(server_computers)/VuLrDEUQ">Tandem/NonStop</a> servers</td><td>The date of the <a href="/facts/Transcontinental_railroad/pcH8GBIl">transcontinental railroad</a> <a href="/facts/Golden_Spike/Se780HwU">Golden Spike</a>, when the <a href="/facts/Central_Pacific_Railroad/WgUDL4qL">Central Pacific</a> and <a href="/facts/Union_Pacific/n9y9FccK">Union Pacific</a> railroads finally met at <a href="/facts/Promontory_Summit,_UT/MNGXTNjs">Promontory Summit, UT</a> in May 1869.</td></tr><tr><td>30 December 1899</td><td><a href="/facts/Variant_type_(COM)/VUo5FpRu">Microsoft COM DATE</a>, <a href="/facts/Object_Pascal/cuGkuISv">Object Pascal</a>, <a href="/facts/LibreOffice/MInj5kF7">LibreOffice Calc</a>, <a href="/facts/Google_Docs,_Sheets_and_Slides/mgEjwvIs">Google Sheets</a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a></td><td>Technical internal value used by Microsoft Excel; for compatibility with <a href="/facts/Lotus_1-2-3/tnkvSG3M">Lotus 1-2-3</a>.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a></td></tr><tr><td>31 December 1899</td><td>Dyalog <a href="/facts/APL_(programming_language)/E2sb98gC">APL</a>,<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> <a href="/facts/Visual_C%2B%2B/RM3sbYw5">Microsoft C/C++ 7.0</a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a></td><td>Chosen so that (date mod 7) would produce 0=Sunday, 1=Monday, 2=Tuesday, 3=Wednesday, 4=Thursday, 5=Friday, and 6=Saturday. Microsoft’s last version of non-Visual C/C++ used this, but was subsequently reverted.</td></tr><tr><td><a href="/facts/0_January/DoFhbkA0">0 January</a> 1900</td><td><a href="/facts/Microsoft_Excel/Ezal6G75">Microsoft Excel</a>,<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> <a href="/facts/Lotus_1-2-3/tnkvSG3M">Lotus 1-2-3</a><a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a></td><td>While logically 0 January 1900 is equivalent to 31 December 1899, these systems do not allow users to specify the latter date. Since 1900 is <a href="/facts/Leap_year_bug/06vgfHzC">incorrectly treated as a leap year</a> in these systems, 0 January 1900 actually corresponds to the historical date of 30 December 1899.</td></tr><tr><td>1 January 1900</td><td><a href="/facts/Network_Time_Protocol/vS21KqT5">Network Time Protocol</a>, <a href="/facts/Time_Protocol/AHl7lgGx">Time Protocol</a>, <a href="/facts/IBM/QszA7nwd">IBM</a> <a href="/facts/CICS/BNRGFat7">CICS</a>, <a href="/facts/Mathematica/dRwoGFG2">Mathematica</a>, <a href="/facts/RISC_OS/Pc96teso">RISC OS</a>, <a href="/facts/ICL_VME/6rICze0T">VME</a>, <a href="/facts/Common_Lisp/mq1pRbSF">Common Lisp</a>, <a href="/facts/Michigan_Terminal_System/4IaHt8CE">Michigan Terminal System</a></td><td></td></tr><tr><td>1 January 1901</td><td><a href="/facts/Ada_(programming_language)/Y2yHel4l">Ada</a><a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a></td><td>In the first version of the language, dates were limited to the range 1901 to 2099 to avoid years affected by the <a href="/facts/Century_leap_year/wthzpnXr">400 year leap year rule</a>. When the upper bound was expanded in later versions, the lower bound was left the same to maintain backward compatibility with systems that used it as a <a href="/facts/Datum_reference/pwGglGnJ">datum</a>.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a></td></tr><tr><td>1 January 1904</td><td><a href="/facts/LabVIEW/ue0vyha4">LabVIEW</a>, <a href="/facts/Apple_Inc./oTJYta3k">Apple Inc.</a>'s <a href="/facts/Classic_Mac_OS/I5fQq44K">classic Mac OS</a>, JMP Scripting Language, <a href="/facts/Palm_OS/nIaAPX4I">Palm OS</a>, <a href="/facts/MP4/PM0FNvUU">MP4</a>, Microsoft Excel (optionally),<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> <a href="/facts/IGOR_Pro/mNlFTCtP">IGOR Pro</a></td><td>1904 is the first leap year of the 20th century.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a></td></tr><tr><td>1 January 1960</td><td><a href="/facts/SAS_System/7y2ulIMX">SAS System</a><a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a></td><td></td></tr><tr><td>31 December 1967</td><td><a href="/facts/Pick_OS/LSXLknzu">Pick OS</a> and variants (jBASE, Universe, Unidata, Revelation, Reality)</td><td>Chosen so that (date mod 7) would produce 0=Sunday, 1=Monday, 2=Tuesday, 3=Wednesday, 4=Thursday, 5=Friday, and 6=Saturday.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a></td></tr><tr><td><a href="/facts/1_January_1970/d8Et1sa2">1 January 1970</a></td><td><a href="/facts/Unix_Epoch/d8Et1sa2">Unix Epoch</a> used in <a href="/facts/POSIX_time/d8Et1sa2">POSIX time</a>, used by <a href="/facts/Unix/0pNL2seI">Unix</a> and <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a> systems (<a href="/facts/Linux/9FvCKSTq">Linux</a>, <a href="/facts/MacOS/Y93vVpTm">macOS</a>, <a href="/facts/Android_(operating_system)/nNyHb0xz">Android</a>), and <a href="/facts/Programming_language/cWTYbgWa">programming languages</a>: most <a href="/facts/C_(programming_language)/Ky2No763">C</a>/<a href="/facts/C%2B%2B/Et0F2qn9">C++</a> implementations,<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> <a href="/facts/Java_(programming_language)/9ScgFyAL">Java</a>, <a href="/facts/JavaScript/XhhIIEsl">JavaScript</a>, <a href="/facts/Perl/fx31kjlT">Perl</a>, <a href="/facts/PHP/vU6YRJX0">PHP</a>, <a href="/facts/Python_(programming_language)/YbuGqofa">Python</a>, <a href="/facts/Ruby_(programming_language)/BeYSaPSR">Ruby</a>, <a href="/facts/Tcl/s7fbKYtR">Tcl</a>, <a href="/facts/ActionScript/xtDQb0wR">ActionScript</a>. Also used by <a href="/facts/Precision_Time_Protocol/N5u5HVqx">Precision Time Protocol</a>.</td><td></td></tr><tr><td>1 January 1978</td><td><a href="/facts/AmigaOS/1E2k1fuN">AmigaOS</a>.<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a><a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a> The Commodore Amiga hardware systems were introduced between 1985 and 1994. Latest OS version 4.1 (December 2016). <a href="/facts/AROS/qctF50SD">AROS</a>, <a href="/facts/MorphOS/xWSeUeWM">MorphOS</a>.</td><td></td></tr><tr><td>1 January 1980</td><td><a href="/facts/IBM_BIOS/AYYPUogS">IBM BIOS</a> INT 1Ah, <a href="/facts/DOS/sWwDXNmX">DOS</a>, <a href="/facts/OS/2/uVBl6ItP">OS/2</a>, <a href="/facts/FAT12/uFvpmWxa">FAT12</a>, <a href="/facts/FAT16/uFvpmWxa">FAT16</a>, <a href="/facts/FAT32/uFvpmWxa">FAT32</a>, <a href="/facts/ExFAT/w9R3QhPC">exFAT</a> filesystems, <a href="/facts/ZIP_(file_format)/qDrrYIB9">ZIP</a> format and derivatives</td><td>The IBM PC with its BIOS as well as <a href="/facts/86-DOS/u0Hm0Ijv">86-DOS</a>, <a href="/facts/MS-DOS/fVrrz1uN">MS-DOS</a> and <a href="/facts/PC_DOS/ScsfqAwQ">PC DOS</a> with their FAT12 file system were developed and introduced between 1980 and 1981.</td></tr><tr><td>6 January 1980</td><td><a href="/facts/Qualcomm/v54S3UgA">Qualcomm</a> <a href="/facts/BREW/9BV8hT0P">BREW</a>, <a href="/facts/GPS/DKVLnzF3">GPS</a>, <a href="/facts/ATSC/RIYWmB7v">ATSC</a> 32-bit time stamps</td><td>GPS counts weeks (a week is defined to start on Sunday) and 6 January is the first Sunday of 1980.<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a><a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> Weeks are stored in a 10-bit integer, and the first <a href="/facts/GPS_week_number_rollover/edXQbWhr">GPS week number rollover</a> occurred in August 1999.</td></tr><tr><td>31 December 1989</td><td><a href="/facts/Garmin/ogjlaRwp">Garmin</a> FIT Epoch.<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a> Standard originated by <a href="/facts/Garmin/ogjlaRwp">Garmin</a> as part of their FIT Protocol, which has been adopted by many as a de facto standard in the fitness devices industry.<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a></td><td></td></tr><tr><td>1 January 2000</td><td><a href="/facts/AppleSingle/mY86Lr5k">AppleSingle</a>, <a href="/facts/AppleDouble/mY86Lr5k">AppleDouble</a>,<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a> <a href="/facts/PostgreSQL/LHdXuHfl">PostgreSQL</a>,<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a><a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a> <a href="/facts/Zigbee/SG0GxpjI">Zigbee</a> UTCTime,<a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a> <a href="/facts/Ingenuity_(helicopter)/EycKDT9X">Ingenuity</a> helicopter<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a></td><td>This Y2K epoch<a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a> is used sometimes to push the 2038 problem to the year 2068, if the processor does not support 64-bit.</td></tr><tr><td>1 January 2001</td><td>NSDate in <a href="/facts/Apple_Inc./oTJYta3k">Apple</a>'s <a href="/facts/Cocoa_(API)/p10p5aFI">Cocoa</a> framework, <a href="/facts/NeXTSTEP/eL0bymga">NeXTSTEP</a></td><td>First day of the third millennium A.D.</td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/System_time/LtoSMX4v">System time</a></li> <li><a href="/facts/Unix_time/d8Et1sa2">Unix time</a></li></ul> <h2 id="notes">Notes</h2> <h2 id="external-links">External links</h2> <ul><li><a href="https://web.archive.org/web/20150907215822/http://www.merlyn.demon.co.uk/critdate.htm">Critical and Significant Dates</a> (J. R. Stockton), an extensive list of dates that are problematic for various operating systems and computing devices.</li> <li><a href="https://web.archive.org/web/20160210030401/http://www.theiet.org/factfiles/it/problem-dates-page.cfm?type=pdf">Potential problem dates for computers (pdf)</a> A list of potential problem dates for computers and software from 2001 to 2100 (IET).</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"[MS-DTYP]: FILETIME". Microsoft Docs. 2020-03-30. <a href="https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-dtyp/2c57429b-fdd4-488f-b5fc-9e4cf020fcdf" target="_blank">https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-dtyp/2c57429b-fdd4-488f-b5fc-9e4cf020fcdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"GregorianCalendar Class". MSDN. Remarks. Retrieved 2015-04-26. <a href="https://msdn.microsoft.com/en-us/library/system.globalization.gregoriancalendar(v=vs.110).aspx" target="_blank">https://msdn.microsoft.com/en-us/library/system.globalization.gregoriancalendar(v=vs.110).aspx</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Spolsky, Joel (2008-02-19). "Why are the Microsoft Office file formats so complicated? (And some workarounds)". Retrieved 2009-03-08. <a href="http://www.joelonsoftware.com/items/2008/02/19.html" target="_blank">http://www.joelonsoftware.com/items/2008/02/19.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Dershowitz, Nachum; Reingold, Edward (2008). Calendrical Calculations (3 ed.). Cambridge University Press. pp. xxi, xxvi. ISBN 978-0-521-70238-6. <a href="978-0-521-70238-6" target="_blank">978-0-521-70238-6</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Subirana, J. Sanz; Zornoza, J. M. Juan; Hernández-Pajares, M. (2011). "Time References in GNSS". gssc.esa.int/navipedia. European Space Agency. <a href="http://www.navipedia.net/index.php/Time_References_in_GNSS" target="_blank">http://www.navipedia.net/index.php/Time_References_in_GNSS</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"GNSS Timescale Description Galileo" (PDF). United Nations Office for Outer Space Affairs. 2016. 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[…] timestamp values are stored as seconds before or after midnight 2000-01-01. <a href="http://www.postgresql.org/docs/9.1/static/datatype-datetime.html" target="_blank">http://www.postgresql.org/docs/9.1/static/datatype-datetime.html</a> <a href="#fnref:48" class="footnote-back-ref">↩</a></p></li> <li id="fn:49"><p>Be aware that the Epoch function return the unix Epoch SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '1970-01-01 00:00:00-00'); returns 0. <a href="#fnref:49" class="footnote-back-ref">↩</a></p></li> <li id="fn:50"><p>"ZigBee Cluster Library Specification". Section 2.5.2.21 UTCTime. <a href="https://www.scribd.com/document/58909727/075366r01ZB-AFG-ZigBee-Cluster-Library-Public-Download-Version" target="_blank">https://www.scribd.com/document/58909727/075366r01ZB-AFG-ZigBee-Cluster-Library-Public-Download-Version</a> <a href="#fnref:50" class="footnote-back-ref">↩</a></p></li> <li id="fn:51"><p>NASA JPL [@NASAJPL] (2021-05-04). 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