What is special about dates before the year 1970?

Question

I see a lot of discussion about getting dates that are pre-1970. For example, I see people ask a question like, "how do I get a date before 1970?"

What I'd like to know is what is so special about 1970? Why do people have trouble getting dates before that particular year? Was it the beginning of the universe or something?

Dan Grossman · Accepted Answer · 2011-02-23T05:07:44.610

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It is the beginning of the UNIX epoch, timestamp 0. All UNIX timestamps are the number of seconds since January 1st 1970 UTC. The moment of this writing is timestamp 1298440626.

UNIX timestamps pop up in the datetime libraries of a lot of languages and software, as storing times as a number of seconds is convenient for various reasons.

Since 1970 is time 0, dates before then can't typically be stored as timestamps.

edited Feb 23 '11 at 05:07

answered Feb 23 '11 at 04:55

Dan Grossman

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Actually, I've seen systems that store almost back to 1900 by taking advantage of the fact it's a signed type. Not overly portable however :-) – paxdiablo Feb 23 '11 at 08:35

score 2 · Answer 2 · answered Feb 23 '11 at 04:56

It has to do with UNIX times. They're strored as number of seconds since the epoch, and the epoch is defined as the start of the day January 1, 1970 (UTC).

That's also the cause for the upcoming Y2K38 bug where the value will roll over to negative sometime early Feb (from memory) in 2038. Unless they up it to beyond a signed 32-bit value, of course.

score 0 · Answer 3 · answered Feb 23 '11 at 04:55

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It was the beginning of the UNIX era.

answered Feb 23 '11 at 04:55

vissi

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Where is "the UNIX era" defined? – Dan Grossman Feb 23 '11 at 05:04
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@Dan Grossman: I suspect he intended to say "UNIX epoch" instead. – Jerry Coffin Feb 23 '11 at 05:07

What is special about dates before the year 1970?

3 Answers3