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by jameshart 4099 days ago
So by epoch, obviously you mean the number of seconds since Jan 1 1970. Midnight - 00:00, right? Ah... but then... UTC? or TAI? There's a 35 second difference, after all. There's a school of thought that the UNIX epoch counts from 1970-01-01 00:00:10 TAI...
2 comments
nknighthb 4099 days ago
POSIX specifies the Epoch to be UTC and has since at least 2001. People may have other opinions on how it should be specified, but if you're going to follow POSIX as it exists, you're not left with a choice in the matter.

http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_...

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jameshart 4098 days ago
But POSIX also claims there are 86400 seconds in a day, which is not always true for UTC. There are two ways of dealing with that - POSIX says that the correct way is to just count seconds, but reset every UTC midnight to (number of days since 1970-1-1) * 86400, which means that when leap seconds occur some epoch numbers are ambiguous (or in a leap-second-deletion, are skipped). NTP ignores POSIX and says that the way to deal with this is to vary the length of a second during a day which contains a leap second.

And we're talking about JSON here, so isn't the ECMA-262 standard for dates more relevant than the POSIX standard? ECMAScript has some very fuzzy ideas about dates.

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nknighthb 4098 days ago
POSIX specifies that there are 86400 seconds in a day. POSIX is not making claims about reality, it is specifying its own reality. That's what standards do.

ECMA-262 isn't really relevant at all, since it's not the (or even an) authority on JSON. JSON was simply derived from it -- in an incompatible way at that. It's doubly irrelevant since you were talking about Unix, so that's what I was addressing.

By the way, your phrasing is odd/confusing. You're talking about "epochs" in a strange way. In Unix/POSIX land, there is one epoch, "The time zero hours, zero minutes, zero seconds, on January 1, 1970 Coordinated Universal Time (UTC).". Unix timestamps are derived from the epoch, they do not define it.

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deitcher 4099 days ago
I had not thought of that. Ha!

I would go for UTC, since that is what 99.99% of the people think of (and so few even know about TAI, except for the smart ones like @jameshart! :-) )

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