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I am having some trouble understanding what exactly 00:00:00 Coordinated Universal Time (UTC), 1 January 1970 means.

Below are two possible ways to interpret 00:00:00 according to my understanding, or rather, according to my confusion:

  • a point in time of zero duration, functioning as a divider between days. Under such a definition, being a divider, it belongs to no day, falls on no day in particular, and is rather defined not by a single day but rather by the two days it divides, so 00:00:00 1 January 1970 doesn't make sense. What would make sense under this definition would be to say 00:00:00 between 31 December 1969 and 1 January 1970, or simply (since that's redundant) the point in time between 31 December 1969 and 1 January 1970.
  • an interval in time having a duration of 1 second starting from the beginning of 1 January 1970 and ending 1 second later. Under the second definition 00:00:00 1 January 1970 makes sense but then the concept of seconds elapsed since 00:00:00 UTC, 1 January 1970 doesn't make sense as you can't measure distance from an interval but only from a point. Unless what we really mean (under this definition) is seconds elapsed since the beginning of 00:00:00 UTC, 1 January 1970 I.e. since the beginning of the interval denoted by 00:00:00 UTC, 1 January 1970.

UPDATE Based on the answers I received I think the correct way to view it is a point in time of zero duration that by convention indicates the earliest point in time in the day next to which it appears. Yet I am not fully convinced that this is a rigorous definition. It's like defining 1 to be the lowest number in the set [1, 2). It's a circular definition as the duration of the day is itself defined using the 00:00:00 point in time. Maybe I am indeed reading too much into it.

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closed as not constructive by Hauke Laging, Anthon, l0b0, Ulrich Dangel, slm Jun 4 '13 at 17:39

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Love the title. – Bruce Ediger Jun 4 '13 at 14:24
What is the question? – l0b0 Jun 4 '13 at 15:14
@IOb0 what are the exact semantics of Unix Epoch if we wish to be pedantic or if we are just plain thick. – Marcus Junius Brutus Jun 4 '13 at 15:23
Isn't 00:00:00 Dec 31, 1969 the second after 23:59:59 Dec 30, 1969? – ott-- Jun 4 '13 at 16:20
up vote 3 down vote accepted

I'm probably missing something here, but I think it's most useful to think of the epoch as a reference. It is instantaneous (somewhat along the lines of your first bullet point when you say zero duration) and is only useful in terms of other dates/times. E.G. 00:00:01 - 00:00:00 = 1 second since epoch.

Think of time as a number line and we choose to place zero at a certain point, since we need a global reference. All points are then relative to this point. Normally, we think of zero in terms of number of things (e.g. "I have zero apples" means "I have no apples"). However, it is common to place zero in a convenient place (think about grid coordinates or energy scales).

The problem you are expressing is far more general than the unix epoch and relates to e.g. 0 AD/BC (CE/BCE).


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You're reading way too much into it.

The epoch is the first moment of time during January 1st 1970 GMT. As one entire second has not yet passed, the time is 00:00:00 .

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