For lots of use cases you want to record precise events in a universal sense, and be able to add and subtract them or compare whether one timestamp is the same exact time as another timestamp regardless of timezones.
UNIX timestamps have no timezones. They are just the number of actual seconds that have elapsed since a certain commonly-agreed time. You measure the number of seconds between two events just by subtracting their timestamps arithmetically, and don't need to do any parsing or interpreting of timezones. You can just say "this hacker in Russia breached the firewall 35 seconds after this dev committed code in the US" just by subtracting two numbers.
The time an airplane takes off, the time your firewall detected a security event, the time a cross-timezone meeting starts, these are all good to store as UNIX timestamps.
Birthdays are different because they are inherently imprecise; humans tend to celebrate their birthdays when it's a certain day of the month in their local time zone, with little to no regard for the time zone they were born in. Someone born in Singapore (UTC+8) and lives in Alaska (UTC-8) might still celebrate their 30th birthday on the month and day of their birthday in Alaska time even though their body might need another 16 hours before it has technically experienced 30 years of life.
There is no such thing as "exact same time" due to relativity. The most perfect of clocks in use on the planet will drift, and both are correct.
Mail an atomic clock easy to west, and a different one west to east, and they'll drift different amounts. Put one in a plane, it will disagree with one on the ground. Put one on a mountain, it will disagree with one at sea level. Put one at the pole, it will differ with one on the equator.
There is little sense in claiming one time is exactly equal to another. Close to some tolerance is useful.
> UNIX timestamps have no timezones. They are just the number of actual seconds that have elapsed since a certain commonly-agreed time. You measure the number of seconds between two events just by subtracting their timestamps arithmetically, and don't need to do any parsing or interpreting of timezones
If only. UNIX mishandling of leap seconds makes this all a illusion, in reality you need to have logic to handle them
I don't think that UNIX mishandles leap seconds. It properly ignores them because they aren't relevant to it. The UNIX timestamp is literally the count of units past the epoch. The existence of leap seconds don't affect that count, it only affects the timekeeping of a completely different scheme.
So yes, you have to handle leap seconds when you're converting from the timestamp to/from the time scheme you are interested in, just as you have to handle leap years, etc. The UNIX timestamp is just a different way of marking time, and is blessed with not worrying at all about how that time relates to the actual motion of the Earth. That makes it easy and consistent for common operations. It is not mishandling anything. It just can't shield you from the complexity of the forms of timekeeping that make it a point to be synchronized with the Earth.
That's not true. Unix time is specifically defined as "A value that approximates the number of seconds that have elapsed since the Epoch."
Are you perhaps confusing that with the standard algorithms for converting between Unix time and "human" time? Because what you say isn't wrong for that case, and this is a classic trap for new players:
"A Coordinated Universal Time name (specified in terms of seconds (tm_sec), minutes (tm_min), hours (tm_hour), days since January 1 of the year (tm_yday), and calendar year minus 1900 (tm_year)) is related to a time represented as seconds since the Epoch, according to the expression below.
If the year is <1970 or the value is negative, the relationship is undefined. If the year is >=1970 and the value is non-negative, the value is related to a Coordinated Universal Time name according to the C-language expression, where tm_sec, tm_min, tm_hour, tm_yday, and tm_year are all integer types:
> That's not true. Unix time is specifically defined as "A value that approximates the number of seconds that have elapsed since the Epoch."
It's missing 27 seconds that elapsed since the epoch. (Plus whatever happened with rubber seconds before 1972). Maybe that fits within the definition of approximate.
But if you need to record events that happen on a leap second or the second before a leap second, unixtime is unhelpful.
If you want to record a duration that includes a leapseconds, unixtime is unhelpful, although if the duration is long enough, the difference may be of no consequence.
Do you think UNIX epoch offset is different for "the second before the leap second", "the leap second" and the "second after the leap second"? If so, how? If not, then we can't rely on differences between UNIX epoch offsets for measuring intervals can we?
Moreover, if the units you are referring to are "seconds" then the statement "[t]he UNIX timestamp is literally the count of units past the epoch" is also wrong. If not, what are the units here?
This might seem like a silly question but could you give me an example use-case?
I'm struggling to think of a context where we basically want to create timestamps for things that happened in the past. Especially things that we want to dynamically localize rather than e.g. just store as a straight up timestamped string.
If you are creating a relationship between times in the past, you have to convert from string to something else. And the something else is always subject to some other detail - the calendar of the past is not defined identically throughout history, so you can't just extrapolate year-month-day backwards to get a time.
Nobody's really solved this, it has to be tackled anew for each case of "comparing dates from long ago".
Postgres's time handling types are elaborate, and probably the best I've personally worked with.
Timestamp with TimeZone (including DST + timezone offset) is a different type from timestamp without TimeZone (ie: normalized to GMT). Timestamp - Timestamp results in "interval".
Perhaps my method I was discussing doesn't work. But the overall idea that "Timestamp" needs to be a different type than "Interval", is pretty key IMO to handling date/times appropriately.
time_t will not handle daylight savings time conversions like Postgresql does.
Unambiguously resolving "Timestamp at Eastern Time - Timestamp at Arizona Time" is not as simple as you might think. Postgresql handles this case.
Trying to make "time_t" handle all cases is a mistake. We need like 3 to 4 types at a minimum to handle all of the edge cases that occur in the real world. (And if anyone teaches me of any more complications, maybe we'll need even more types)
The idea is to store all times as time_t in UCT, and do all time calculations in UTC.
Do conversions to/from local time only as a first and last step.
It's the only sane way to do it. The D runtime library does that, and it has proven to be robust and sane. We're also recommending that users do the same thing with different character representations - do all calculation and processing in UTF-8. Other sets are translated upon input into UTF-8, and the UTF-8 is translated to other character sets only on output.
> Do conversions to/from local time only as a first and last step.
Cool.
Now how do you enforce that? Spoiler: have the local-time be reported as a TimestampWithTimeZone.
When you're doing calculations, make them all Timestamps.
When you're done, you can remember that you've converted from internal UTC-timestamps into the proper output format because you'll have a TimestampWithTimezone again.
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We're programmers. We have problems that are simple and enforceable through compiler constructs called "The Type System".
Why have the human figure out if they have time-zone back-and-forth correctly or otherwise made a mistake (ex: adding code to the wrong "part of the process" and accidentally mucking with a TimestampWithTimeZone input/output variable rather than the internal Timestamp-UTC-only type variables?)
Just... leave the job of checking this entire process to the compiler. If your n00b coworker messes up, the type system catches it and we're all set.
UNIX timestamps have no timezones. They are just the number of actual seconds that have elapsed since a certain commonly-agreed time. You measure the number of seconds between two events just by subtracting their timestamps arithmetically, and don't need to do any parsing or interpreting of timezones. You can just say "this hacker in Russia breached the firewall 35 seconds after this dev committed code in the US" just by subtracting two numbers.
The time an airplane takes off, the time your firewall detected a security event, the time a cross-timezone meeting starts, these are all good to store as UNIX timestamps.
Birthdays are different because they are inherently imprecise; humans tend to celebrate their birthdays when it's a certain day of the month in their local time zone, with little to no regard for the time zone they were born in. Someone born in Singapore (UTC+8) and lives in Alaska (UTC-8) might still celebrate their 30th birthday on the month and day of their birthday in Alaska time even though their body might need another 16 hours before it has technically experienced 30 years of life.