Why is 1 1 1970 the epoch time

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Why is     1 January 1970 00 00 00   considered the epoch time

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History      The earliest versions of Unix time had   a 32-bit integer incrementing at a   rate of 60 Hz  which was the rate of   the system clock on the hardware of   the early Unix systems  The value 60   Hz still appears in some software   interfaces as a result  The epoch also   differed from the current value  The   first edition Unix Programmer s Manual   dated November 3  1971 defines the   Unix time as  the time since 00 00 00    Jan  1  1971  measured in sixtieths of   a second

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Short answer  Why not   Longer answer  The time itself doesn t really matter  as long as everyone who uses it agrees on its value  As 1 1 70 has been in use for so long  using it will make you code as understandable as possible for as many people as possible   There s no great merit in choosing an arbitrary epoch just to be different

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Epoch reference date  An epoch reference date is a point on the timeline from which we count time  Moments before that point are counted with a negative number  moments after are counted with a positive number    Many epochs in use     Why is 1 January 1970 00 00 00 considered the epoch time    No  not the epoch  an epoch  There are many epochs in use   This choice of epoch is arbitrary    Major computers systems and libraries use any of at least a couple dozen various epochs  One of the most popular epochs is commonly known as Unix Time  using the 1970 UTC moment you mentioned    While popular  Unix Time   s 1970 may not be the most common  Also in the running for most common would be January 0  1900 for countless Microsoft Excel  amp  Lotus 1-2-3 spreadsheets  or January 1  2001 used by Apple   s Cocoa framework in over a billion iOS macOS machines worldwide in countless apps  Or perhaps January 6  1980 used by GPS devices   Many granularities  Different systems use different granularity in counting time    Even the so-called    Unix Time    varies  with some systems counting whole seconds and some counting milliseconds  Many database such as Postgres use microseconds  Some  such as the modern java time framework in Java 8 and later  use nanoseconds  Some use still other granularities    ISO 8601  Because there is so much variance in the use of an epoch reference and in the granularities  it is generally best to avoid communicating moments as a count-from-epoch  Between the ambiguity of epoch  amp  granularity  plus the inability of humans to perceive meaningful values  and therefore miss buggy values   use plain text instead of numbers   The ISO 8601 standard provides an extensive set of practical well-designed formats for expressing date-time values as text  These formats are easy to parse by machine as well as easy to read by humans across cultures   These include     Date-only  2019-01-23 Moment in UTC   2019-01-23T12 34 56 123456Z Moment with offset-from-UTC  2019-01-23T18 04 56 123456 05 30 Week of week-based-year  2019-W23 Ordinal date  1st to 366th day of year   2019-234

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Early versions of unix measured system time in 1 60 s intervals  This meant that a 32-bit unsigned integer could only represent a span of time less than 829 days  For this reason  the time represented by the number 0  called the epoch  had to be set in the very recent past  As this was in the early 1970s  the epoch was set to 1971-1-1   Later  the system time was changed to increment every second  which increased the span of time that could be represented by a 32-bit unsigned integer to around 136 years  As it was no longer so important to squeeze every second out of the counter  the epoch was rounded down to the nearest decade  thus becoming 1970-1-1  One must assume that this was considered a bit neater than 1971-1-1   Note that a 32-bit signed integer using 1970-1-1 as its epoch can represent dates up to 2038-1-19  on which date it will wrap around to 1901-12-13

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http   en wikipedia org wiki Unix time History explains a little about the origins of Unix time and the chosen epoch  The definition of unix time and the epoch date went through a couple of changes before stabilizing on what it is now  But it does not say why exactly 1 1 1970 was chosen in the end  Notable excerpts from the Wikipedia page   The first edition Unix Programmer s Manual dated November 3  1971 defines the Unix time as  quot the time since 00 00 00  Jan  1  1971  measured in sixtieths of a second quot   Because of  the  limited range  the epoch was redefined more than once  before the rate was changed to 1 Hz and the epoch was set to its present value  Several later problems  including the complexity of the present definition  result from Unix time having been defined gradually by usage rather than fully defined to start with

[unix] Why is 1/1/1970 the "epoch time"?

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