Do UNIX timestamps change across timezones? - timezone

As the subject asks; do UNIX timestamps change in each timezone?
For example, if I sent a request to another email the other side of the world saying, "Send out an email when the time is 1397484936", would the other server's timestamp be 12 hours behind my own?

The definition of UNIX timestamp is time zone independent. The UNIX timestamp is the number of seconds (or milliseconds) elapsed since an absolute point in time, midnight of Jan 1 1970 in UTC time. (UTC is Greenwich Mean Time without Daylight Savings time adjustments.)
Regardless of your time zone, the UNIX timestamp represents a moment that is the same everywhere. Of course you can convert back and forth to a local time zone representation (time 1397484936 is such-and-such local time in New York, or some other local time in Djakarta) if you want.
The article at http://en.wikipedia.org/wiki/Unix_time is pretty impressive if you'd like a longer read.

Unix time is defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970. So the answer is no

Unix timestamps do not change accross timezones, they are created for the purpose of having a standard time across globe.
NOTE:-
Timestamps are calculated on the basis of current time in the computer thus do not rely on them until and unless you are very sure about the time settings in the participating machines.

Someone stated that "UTC is Greenwich Mean Time without Daylight Savings time adjustments." This is simply untrue. GMT does not have Dayllight Savings Time. GMT is measured in Greenwich, England (at the Naval Obeservatory) [0 longitude, but not 0 lattitude]. UTC is measured at the equator [0 longitude and 0 lattitude - which happens to lie in the ocean off the cost of Africa].
What difference does it make? It doesn't make a difference in terms of "what time of day is it?" It does, however, make a difference in terms of calculating a year. Now you'd think a year would be measured based upon the location of the center (the core) of the earth, right? When the earth's core is back in the same location it was ~365 days ago, it has been a year. It isn't measured that way. It is measured by a specific location on the earth getting back to the same location (relative to the sun) that it was ~365 days ago. But the period of a day and a year don't divide evenly. Once the earth is back to about where it was a year ago, the earth isn't facing the same direction it was last year, so that spot on the earth isn't facing the same direction it was a year ago. Being further north, Greenwich isn't going to get back to the same spot (relative to the sun) that it was last year at the same time that 0 Lat / 0 Long is. So if you base the definition on Greenwith vs. 0/0, you get a, albeit slightly, different answer to the question "how many days are in a year". To put it another way, when a given spot on the earth gets back to where it was a year ago (relative to the Sun), the core of the earth isn't in the same spot it was a year ago, so what spot you pick matters because the core of the earth is going to be in a different spot (relative to the sun) than it was one year ago, if you pick a different spot on the earth.
Neither UTC nor GMT have daylight savings time. Europe/London time, the timezone that Greenwich resides in, does. But GMT does not. GMT is, what Americans would call a "Standard Time" - i.e. without DST.
Getting back to the question, Epoch time doesn't technically have a timezone. It is based on a particular point in time, which just so happens to line up to an "even" UTC time (at the exact beginning of a year and a decade, etc.). If that concept doesn't fit well in your brain, and if it helps to think of Epoch time as being in UTC, go right ahead. You're in good company and in the grand scheme of things, it really doesn't matter. You ever see those law suits where somoene is awarded $1. It's kind of a "you're right, but it doesn't really matter" type of verdict. If someone sued you for saying Epoch time is in the UTC timezone, they would win $1. That wouldn't buy them a cup of coffee at any Starbucks in any timezone on the planet.

IF both computers are set up correctly with their clocks set for the correct timezone and UTC values, they should return the same value.
Of course that's a big IF. There's almost certain to be a difference of at least a second, more often minutes between the time reported by two computers. And many computers are set up to have incorrect timezone settings, and will report their local time when asked a timestamp rather than UTC.
And in that lies the difference between theory and practice. In theory it's all the same, in practice you should not rely on it.

No, epoch timestamp should not change, because it has a fixed timezone which is UTC.
If you want to use a time object in other time zone, just look it up in libraries of the language you use, but do NOT try to add/substract a couple of hours from epoch timestamp and assume it's in another time zone, which will make things very confusing to other people, especially when you expose it in your API.
If you use C++, I recommend this library. I heard it will soon be added into standard library.
For all, I understand sometimes time object is hard to deal with and it looks easier to add/substruct on epoch timestamp. Please don't do it and do not persuade others to do it. A time object is much easier once you get used to it and can take care of time zone conversion easily without messing up with historical time zone changes due to politics/law etc...

Related

Is date in the target timezone in ISO 8601?

I trying to parse a date in ISO 8601 format and some moments aren't clear for me.
For example, I have the next date: 2020-04-16T07:16:34.858215+03:00 in Europe/Moscow timezone.
Does it mean 07:16 in Moscow time or 10:16? I mean do I need to add 3 hours to date or date is in Moscow time already and timezone just shows how it diffs from UTC?
P.S. I tried to find information about it but everywhere is just common format description without details.
The time specified is the local time, so 2020-04-16T07:16:34.858215+03:00 means 7:16am in Moscow, or 4:16am UTC.
Wikipedia has a good example which clarifies things to at least some extent:
The following times all refer to the same moment: "18:30Z", "22:30+04", "1130−0700", and "15:00−03:30". Nautical time zone letters are not used with the exception of Z. To calculate UTC time one has to subtract the offset from the local time, e.g. for "15:00−03:30" do 15:00 − (−03:30) to get 18:30 UTC.
It's really unfortunate that ISO-8601 talks about this as a time zone, when it's only a UTC offset - it definitely doesn't specify the actual time zone. (So you can't tell what the local time will be one minute later, for example.)

Is timezone just an offset number or "more information"?

I live in a country where they change the time twice a year. That is: there is a period in the year when the offset from UTC is -3 hours (-180 mins) and other period where the offset is -4 hours (-240 mins)
Grafically:
|------- (offset = -3) -------|------- (offset is -4) -------|
start of year mid end of year
My question is:
the "timezone" is just the number representing the offset? that is: my country has two timezones? or the timezone includes this information?
This is important because I save every date in UTC timezone (offset = 0) in my database.
Should I, instead, be saving the dates with local timezone and saving their offset (at the moment of saving) too?
Here is an example of a problem I see by saving the dates with timezone UTC:
Lets say I have a system where people send messages.
I want to have a statistics section where I plot "messages sent v/s hour" (ie: "Messages sent by hour in a regular day")
Lets say there are just two messages in the whole database:
Message 1, sent in march 1, at UTC time 5 pm (local time 2 pm)
Message 2, sent in august 1, at UTC time 5 pm (local time 1 pm)
Then, if I create the plot on august 2, converting those UTC dates to local would give me: "2 messages where sent at 1 pm", which is erratic information!
From the timezone tag wiki here on StackOverflow:
TimeZone != Offset
A time zone can not be represented solely by an offset from UTC. Many
time zones have more than one offset due to "daylight savings time" or
"summer time" rules. The dates that offsets change are also part of
the rules for the time zone, as are any historical offset changes.
Many software programs, libraries, and web services disregard this
important detail, and erroneously call the standard or current offset
the "zone". This can lead to confusion, and misuse of the data. Please
use the correct terminology whenever possible.
There are two commonly used database, the Microsoft Windows time zone db, and the IANA/Olson time zone db. See the wiki for more detail.
Your specific questions:
the "timezone" is just the number representing the offset? that is: my country has two timezones? or the timezone includes this information?
You have one "time zone". It includes two "offsets".
Should I, instead, be saving the dates with local timezone and saving their offset (at the moment of saving) too?
If you are recording the precise moment an event occurred or will occur, then you should store the offset of that particular time with it. In .Net and SQL Server, this is represented using a DateTimeOffset. There are similar datatypes in other platforms. It only contains the offset information - not the time zone that the offset originated from. Commonly, it is serialized in ISO8601 format, such as:
2013-05-09T13:29:00-04:00
If you might need to edit that time, then you cannot just store the offset. Somewhere in your system, you also need to have the time zone identifier. Otherwise, you have no way to determine what the new offset should be after the edit is made. If you desire, you can store this with the value itself. Some platforms have objects for exactly this purpose - such as ZonedDateTime in NodaTime. Example:
2013-05-09T13:29:00-04:00 America/New_York
Even when storing the zone id, you still need to record the offset. This is to resolve ambiguity during a "fall-back" transition from a daylight offset to a standard offset.
Alternatively, you could store the time at UTC with the time zone name:
2013-05-09T17:29:00Z America/New_York
This would work just as well, but you'd have to apply the time zone before displaying the value to anyone. TIMESTAMP WITH TIME ZONE in Oracle and PostgreSQL work this way.
You can read more about this in this post, while .Net focused - the idea is applicable to other platforms as well. The example problem you gave is what I call "maintaining the perspective of the observer" - which is discussed in the same article.
that is: my country has two timezones? or the timezone includes this information?
The term "timezone" usually includes that information. For example, in Java, "TimeZone represents a time zone offset, and also figures out daylight savings" (link), and on Unix-like systems, the tz database contains DST information.
However, for a single timestamp, I think it's more common to give just a UTC offset than a complete time-zone identifier.
[…] in my database.
Naturally, you should consult your database's documentation, or at least indicate what database you're using, and what tools (e.g., what drivers, what languages) you're using to access it.
Here's an example of a very popular format for describing timezones (though not what Windows uses).
You can see that it's more than a simple offset. More along the lines of offsets and the set of rules (changing over time) for when to use which offset.

Converting to UTC with known timezone offset

Not sure if the title of my question was accurate so sorry if it's misleading, here goes.
I am doing some work with that involves timezones and i just want to make sure i get this right... if i want something to start at 03:00:00 my time and my timezone offset is -5 all i need to do is add 5 to 03:00:00 giving me 08:00:00 and that is the UTC time?
It depends what you mean by "timezone offset". Usually an offset is expressed as the amount added to UTC to get to local time, in which case you need to subtract it from the local time in order to get back to UTC (so it would be 22:00 on the previous day in your case).
So for example, Pacific Daylight Time has an offset of -7 - it's 7 hours behind UTC.
However, there are situations (annoyingly) where the offset is expressed the other way round, so make sure you know which way is appropriate for your specific context.
Note that knowing the offset doesn't mean you know the time zone - there can be multiple time zones with the same offset for a particular moment, but different rules for when the offset changes.
...Yes. Depending on what language you're doing it in, it may or may not be as easy as writing
time-offset;

How is the result of the Delphi NOW function affected by the option to "Adjust clock for daylight saving changes"

I realise this question could have been answered by writing some test code. I'm not lazy, I just thought that the answer might be generally useful.
I have an app that has generated a large amount of data with records that were stamped with the local time (as returned by the NOW routine). We have run into a snag with transitions in and out of daylight savings time - namely that there is an hour missing when we change to DST, and an hour repeated when we exit from DST. This causes problems with manipulations that assume date ordered records.
The app has been altered therefore to work with all datetimes in UTC, but I will have the ability to display datetimes in UTC or in local time. I also have to deal with datetimes that were stored in local time, and make sure they are correctly shifted to UTC. This is tricky, as the datetime might have been stored while DST was in effect, so in the general case I need to be able to determine if any random date is within or outside a DST period. There is of course a period of one hour where a datetime is ambiguous and could be in the last hour before daylight savings ended, or in the first hour after it ended. There is no way of resolving this.
In coding the changes, I wondered about the result of NOW calls. Internally it calls GetLocalTime. What does GetLocalTime (and NOW) return when you are inside a DST period, but the option to "Adjust clock for daylight saving changes" is turned off?
How do I write a routine that returns the current datetime inside a DST period (with the DST bias applied) regardless of whether "Adjust clock for daylight saving changes" is off or on?
I don't think you can solve your problem easily.
There are too many variables:
the stored timestamp
the time zone you are in
the ever changing time zone rules
confirmation that these time zone rules are accurate on all the equipment you use (i.e. everyone always applied their patches)
the inaccuracy of your clock
There is a Delphi TZDB project that can help you with the time zone rules.
I think it is much more practical to not rely on all the above variables, but store three fields:
the timestamp in your local format
the current timezone
the timestamp in UTC format
You perform the sorting on the third field, and the first two fields for displaying.
--jeroen
Use TzSpecificLocalTimeToSystemTime (and its obvious inverse). These allow you to convert between UTC and local date/time based on the daylight savings settings in effect at the local date/time. If you want your app to run on anything earlier than XP, load this (from kernel32) with the 'delayed' function attribute:
function TzSpecificLocalTimeToSystemTime(lpTimeZoneInformation: PTimeZoneInformation;
var lpLocalTime, lpUniversalTime: TSystemTime): BOOL; stdcall;
function TzSpecificLocalTimeToSystemTime; external kernel32 name 'TzSpecificLocalTimeToSystemTime' delayed;

What is the common practice with regards to differentiating between UTC and GMT?

I finally found out the difference between UTC and GMT by making the effort to look it up on Wikipedia today. Technically speaking it appears that GMT != UTC because you do not know if it is UTC or UT1 being referred to. However practically, people use the terms interchangeably to indicate the same timezone.
A while ago, I suggested that we change the user interface of one of my companies apps to display UTC instead of GMT.
Just to be sure that our database was not calculating the potential seconds difference between GMT and UTC, I ran the below query and verified that they both are just acting as aliases for the same timezone.
select now() AT TIME ZONE 'GMT', now() AT TIME ZONE 'UTC';
timezone | timezone
----------------------------+----------------------------
2009-02-11 08:46:11.643032 | 2009-02-11 08:46:11.643032
(1 row)
What do you think? Do enough users out there understand UTC? Is it better to use the older but more common term? Or should I just do a UTC/GMT?
Normal humans don't need to worry about the few seconds difference between GMT and UTC. The difference only matters to astronomers and time nerds.
I have seen very little software that bothers to make the distinction. Most software ends up using the labels "GMT" and "UTC" interchangeably. Typically it just means "clock time after removing the local time zone offset in exact hours (or half/quarter hours)."
In most cases, nobody will be concerned about the sub-second technical difference between GMT and UTC.
However, writing that the time is expressed in UTC instead of GMT avoids one source of confusion:
Greenwich (and the UK in general) is currently GMT+01:00 because of the daylight saving time (DST).
GMT+01:00 does not mean 1 hour ahead of the time in the UK as one could mistakenly think. Because of the DST, GMT+01:00 is currently the exact time in England.
Stating it as UTC+01:00 helps to avoid this confusion.
Personally, I think of the term UTC before I think of GMT.
I think of GMT before UTC, but I am also living at GMT (+/-0)

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