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

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.

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.)

Rails, Moment, and Timezones (abbreviations + offsets)

I am creating a Rails API that will be consumed by a Javascript framework. Time display and manipulation will be controlled with MomentJS. It is important for the front-end to be able to display the dates along with the time zone abbreviations (e.g. 1/1/2010 11:00 PST).
From what I understand, an offset (e.g. -0700) is not enough to determine the actual timezone, and the timezone abbreviations aren't always unique.
I can think of only two options to solve this:
Return all times in UTC and have an extra field for each time specifying the timezone (e.g. { pick_up: "17-06-08T18:59:21.215Z", pick_up_tz: 'America/New_York' } (or pick_up_tz: 'PST')
Use a non-standard datetime format, something that includes both the timezone abbreviation and the offset (e.g. { pick_up: "17/09/06 13:34:00 CDT -05:00")
Are these reasonable solutions or is there a better way?
Use a non-standard datetime format
Never do this. An API should always return time in ISO 8601.
Return all times in UTC and have an extra field for each time specifying the timezone
You are correct that an offset is insufficient to identify time zone, so you do need to include that in the API response, ideally as an IANA time zone.
Whether you convert to UTC or include an offset is a matter of preference; 2017-06-08T18:59:21.215Z and 2017-06-08T11:59:21.215-0700 mean the same time, regardless of the time zone you convert to for display. Including the offset can be useful to identify if the stored offset is different from the time zone, as you might only show the time zone qualifier if the offset is different.

Do UNIX timestamps change across timezones?

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...

How would I add support for timezone in my application?

We're currently storing local datetime of Pacific/Hawaii in the database. Assuming that I cannot change these dates to UTC, what information do I need to add to support timezone?
My thoughts are:
First, add a timezone field to indicate which timezone the user is viewing from. (The user will select this from a dropdown)
Second, add timezone field to indicate the timezone (Pacific/Hawaii) of the current datetimes values in the database.
Third, add offset to cover DST hours
So say a user from America/Los_Angeles views the site, it would pull the datetime from the database, append the offset and apply the timezone of Hawaii before converting it to Los Angeles time. For any calculation or comparison I would convert the Hawaii time to UTC first, then convert the UTC result to Los Angeles. Am I missing anything?
Your question is very broad, and without knowing more about your application, the platform, how you use collect dates and times, what they represent, etc., I can only speak in generalities.
Storing in UTC is recommended, but that is just by convention. The main necessities are that the time zone you are storing data in does not have DST (which Hawaii hasn't since 1947), and that you do not rely on your computer's operating system or environment settings to determine what time zone to use. You can use the Hawaiian time zone if you must. Be sure you document it somewhere though! It will surely be a surprise to anyone else that comes along in the lifecycle of the application.
While it would wolk, there is absolutely no advantage to doing this. You could just as easily convert your data to UTC when you roll out these changes and use UTC going forward. (That would be the preferred approach.)
The IANA time zone ID for Hawaii is "Pacific/Honolulu". If you're on Windows/.Net, the TimeZoneInfo ID is "Hawaiian Standard Time". Either way, they must be spelled, cased, and punctuated in exactly that manner.
Make sure you understand that a Time Zone Offset and a Time Zone are two different concepts. While Hawaii may use a fixed offset of -10:00, that's not guaranteed for most time zones. Please read the timezone tag wiki for further details.
You should probably not attempt to implement your own time zone logic. There are libraries for this in almost every language. Look to see what is appropriate for your platform. (If you provide details, I can offer suggestions.)
It would be a more robust solution to store the times as UTC time as time zone is local to the specific PC that is displaying the data. In your case if you store time plus offset how can you decide which offset to store? Not a workable solution if multiple time zones are involved.

How to get the timezone name from numeric timezone value in Android

I am basically trying to read a .vcs file in Android. It has timezone value in the below format:
TZ:+05:30
Now I want to get the timezone name corresponding to this value. Means in this case it would be Kolkata(India).
Is there any code to achieve this in android?
TimeZone tz = TimeZone.getTimeZone("GMT-06:00");
String tzarr[] = TimeZone.getAvailableIDs(tz.getRawOffset());
for(int i=0;i<tzarr.length;i++)
(I'm assuming you can parse the text into an offset easily enough.)
In general, you can't. Something like "+05:30" just represents an offset from UTC at one particular time. It doesn't express how the offset changes within the time zone across the year (or across history). For example, for some of the time the Europe/London time zone has the same offset as Africa/Casablanca - but not always.
Assuming this is associated with a specific date/time, you could use TimeZone.getAvailableIDs, iterate over all the time zones, check what the offset from UTC is at that particular instant (using TimeZone.getOffset(long)) and see which time zones have the right offset at the right time. There could be many such zones though.
If you don't have a specific date/time, it's even more ambiguous. You can use getRawOffset and getDSTSavings to see whether the target offset is either the standard or DST offset for any particular zone - although note that these calls assume that for a particular time zone, the DST offset and standard offset remain the same across history (which isn't always true).

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