i have this problem, there is a system thats has the duty to deliver a message in the future in a certain date, like a time limit, so this system is in another country thats have the time zone -4 hours and the country thats recibe the message is in a time zone -3 hours, for this country are Chile and Brazil, the problem is, this countries when they enter to the summer time they add one hour, but in different dates, for a period of time like 4 months thy have the same time zone, so for me to deliver the message in the right time i have to add one hour to the date in when the time zone are in -4 and - 3 but when the time zone match -3 hours i dont have to do nothing.
for this task i want to use the function time.LoadLocation("America/Sao_Paulo"), but reading the documentation dosen't mention if they fix the time zone when this countries change their time zone.
does anyone know if they adjust the time zone, or know another function thats can work in this situation?
Areas that observe daylight savings time don’t change the time zone, they change the time standard. For instance, Los Angeles observes a given time eight hours before UTC for much of the year, but seven hours before UTC during the summer; yet Los Angeles is always “Pacific time”.
If you set the time zone to "America/Sao_Paulo", times will be measured relative to Sao Paulo’s then-active time standard.
Related
What is the definition of Daylight Savings Time 'Overlap' & 'Gap'?
I have a hazy understanding of them, so I'd like to confirm... What does it mean to be "within" either of them?
What does it mean to "correct" for DST Gap or DST Overlap? When does a time need correcting, and when does it not need correcting?
The above questions are language-agnostic, but an example of their application I have is:
When to call org.joda.time.LocalDateTime#correctDstTransition?
Correct date in case of DST overlap.The Date object created has
exactly the same fields as this date-time, except when the time would
be invalid due to a daylight savings gap. In that case, the time will
be set to the earliest valid time after the gap. In the case of a
daylight savings overlap, the earlier instant is selected.
Much of this is already explained in the DST tag wiki, but I will answer your specific questions.
What is the definition of Daylight Savings Time 'Overlap' & 'Gap'?
...
What does it mean to be "within" either of them?
When daylight saving time begins, the local time is advanced - usually by one hour. This creates a "gap" in the values of local time in that time zone.
For example, when DST starts in the United States, the clocks tick from 1:59 AM to 3:00 AM. Any local time value from 2:00 AM through 2:59 AM would be considered to be "within the gap".
Note that values in the gap are non-existent. They do not occur in the real world, unless a clock was not correctly advanced. In practice, one typically gets to a value within the gap by adding or subtracting an elapsed time value from another local time.
When daylight saving time ends, the local time is retracted by the same amount that was added when it began (again, usually 1 hour). This creates an "overlap" in the local time values of that time zone.
For example, when DST ends in the United states, the clocks tick from 1:59 AM back to 1:00 AM. Any local time value from 1:00 AM through 1:59 AM is ambiguous if there is no additional qualifying information.
To be "within the overlap" means that you have a value that is potentially ambiguous because it falls into this range.
Such values may belong to the daylight time occurrence (which comes first sequentially), or may belong to the standard time occurrence (which comes second sequentially).
What does it mean to "correct" for DST Gap or DST Overlap?
Correcting for the gap means to ensure that the local time value is valid by possible moving it to a different value. There are various schemes in use for doing so, but the most common and most sensible is to advance the local time value by the amount of the gap.
For example, if you have a local time of 2:30 AM, and you determine it to occur on the day of the spring-forward transition in the United States, then it falls into the gap. Advance it to 3:30 AM.
This approach tends to work well because simulates the act of a human manually advancing an analog clock - or rather, correcting with the idea that it had not been properly advanced.
Correcting for the overlap means to ensure that all local times are well qualified. Usually this is accomplished by assigning a time zone offset to all values.
In the case of a value that is not ambiguous, the offset is deterministic.
In the case of a value that falls within the overlap on the day of a fall-back transition, it often makes sense to choose the first of the two possible values (which will have the daylight time offset). This is because time moves in a forward direction. However, there are sometimes cases where it makes sense to use a different rule, so YMMV.
When does a time need correcting, and when does it not need correcting?
If you are attempting to work with time as an instantaneous value, such as determining the elapsed duration between two values, adding an elapsed time to a specific value, or when converting to UTC, then you need to correct for gaps and overlaps as they occur.
If you are only working with user input and output, always displaying the exact value a user gave you (and never using it for math or time zone conversions) then you do not need to correct for gaps and overlaps.
Also, if you are working with date-only values, or time-only values, then you should not be applying time zone information at all, and thus do not need to correct for gaps and overlaps.
Lastly, if you are working strictly with Coordinated Universal Time (UTC), which has no daylight saving time, then you do not need to correct for gaps and overlaps.
When to call org.joda.time.LocalDateTime#correctDstTransition?
You don't. That method is private, and is called by other Joda-time functions as needed.
I've a epoch time it's seconds or milliseconds format. How do I get time zone and day light saving details from that epoch time? Or do I have to pass as a separate value?
An epoch time in second or millisecond resolution is just a number. And a number cannot hold more than ONE information, here the elapsed time since an epoch.
Time zone or daylight saving details must therefore be transmitted as extra information. Keep also in mind that a time zone is often pretty complex (has an ID, a name, a history of offset transitions etc.) so most time zones cannot just be expressed as simple numbers.
By the way, I cannot give more concrete answer how to transmit extra details like time zones since you have not even told us which programming language or tool you use.
i need a correct weekly ISO 8601 recurring time interval.
For example, repeat:
Each Monday, 19:00.
I already tried a lot of examples, but nothing worked correctly :-(
Thx for any help!
R/2014-W01-1T19:00:00/P1W
Represents a unbounded recurrence with a start date and a duration of one week. The recurrence starts at the first day (Monday) of the week number 01 in the week year 2014. I have used a week date, but you can substitute it with a calendar date or ordinal date, just make sure the date falls on a Monday.
I understand that it's pretty easy to display local time to a user given an offset from GMT for the timezone (e.g. -7 hours for Pacific Daylight Time).
But, what if I want to continue to display the correct local time for a long period of time (say 1 year). Given just an offset from GMT, you do not know what timezone the user is in. E.g. given -7, the user may live in the US or in Canada (or in some other country). These countries my have different local times at different points of the year (e.g. if the US changes to daylight time in March and CA in April).
My question is, is the above paragraph correct? Is there a standard way to take a GMT offset and make a good guess as to which timezone the user is in?
Your conclusion is correct: There is no reliable way to identify a time zone from an offset-from-UTC.
For your example of an offset of -07:00, I count about three dozen possible time zones in the current list including: America/Boise, America/Chihuahua, and America/Edmonton.
A time zone is actually a collection of offsets, recording changes made over time with a certain offset used in that region for a certain period of time but then changed over another certain period of time.
For example, in America/Los_Angeles part of each year has an offset of -08:00 while another part of the year has -07:00. So that time zone is accumulating a history of at least two offsets per annum. In contrast, the neighboring region using the time zone America/Phoenix is not accumulating changes in its offset, having retained the same -07:00 offset for decades.
So we have a many-to-many relationship between offsets and time zones. An offset may appear in one or more time zones. And each time zone can have one or more offsets (having changed over history).
Include time zone name
This is why the designers of the java.time class ZonedDateTime have taken the liberty of having its toString method extend the standard ISO 8601 format using only offsets to also append the name of the time zone in square brackets.
For example, instead of simply generating the standard format: 2007-12-03T10:15:30+01:00, the class generates 2007-12-03T10:15:30+01:00[Europe/Paris] with the name of the zone Europe/Paris appended.
ZonedDateTime.now( ZoneId.of( "America/Montreal" ) )
.toString()
2007-12-03T10:15:30+01:00[Europe/Paris]
I hope this practice of appending the time zone name catches on. The lack of time zone name is a surprising omission by the standards committees that otherwise did an outstanding job in designing ISO 8601.
You can guess about the time zone using clues from the context of your business scenario. But I advise against that. Guessing is risky, especially because politicians around the world have a penchant for frequently re-defining time zones.
UTC
The best practice in storing, serializing, and exchanging date-time values is generally to adjust into UTC. Assuming a date-time library’s tzdata is up-to-date, adjusting into UTC provides a reliable value that is always correct and unambiguous.
In Java, for example, that means using or extracting an Instant object. The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds (up to nine (9) digits of a decimal fraction).
Instant instant = Instant.now();
…or…
ZonedDateTime zdt = ZonedDateTime.now( ZoneId.of( "America/Montreal" ) );
Instant instant = zdt.toInstant();
The standard ISO 8601 formatted string representing such a value uses a Z as short for Zulu and meaning UTC.
2007-12-03T09:15:30Z
OffsetDateTime
If you are given a string representing a date-time with only an offset, parse it as a OffsetDateTime object.
OffsetDateTime odt = OffsetDateTime.parse( "2007-12-03T10:15:30+01:00" );
odt.toString(): 2007-12-03T10:15:30+01:00
From there you can extract a value in UTC, an Instant.
Instant instant = odt.toInstant();
instant.toString(): 2007-12-03T09:15:30Z
Or you can adjust into a desired time zone to get the same moment as a ZonedDateTime object.
ZoneId z = ZoneId.of( "Asia/Kolkata" );
ZonedDateTime zdt = odt.atZoneSameInstant( z );
zdt.toString(): 2007-12-03T14:45:30+05:30[Asia/Kolkata]
Terminology: “local”
I suggest you avoid the term “local” when referring to a date-time adjusted into a region’s particular wall-clock time. The word “local” in the java.time classes and in other contexts means any locality, rather than a particular locality. A local date-time is not a specific moment on the timeline, only a rough idea about a range of possible moments.
For example the local date-time of the beginning of Christmas this year is 2017-12-25T00:00:00, but that moment of midnight is much earlier in Auckland than in Kolkata, and still hours later in Paris, while even more hours later in Montréal.
I suggest instead you use the terms zoned or wall-clock time when you mean a specific moment seen through the lens of a particular time zone.
Tip: Offset literals
The Question happened to use the letters -7 when referring to an offset. I suggest always:
Using double-digits with padding zero, as required by ISO 8601
Using both hour and minutes, as expected by some libraries and formats.
So use -07:00 rather than -7.
I'm looking for a better than O(n) algorithm to determine if a date in the future will have daylight savings time applied (and how much). Given a year, month, day, hour, minute and time zone (and a copy of the Olsen Time Zone database) how does one efficiently determine if that date will be in DST? I'm looking for the algorithm, not a library function to call.
Thank you.
FURTHER EXPLANATION: The date library I'm using is very slow when you create an object with a date in the future and a time zone. It turns out its doing a linear calculation to calculate if the date is in daylight savings time. Not only that, its doing this at object creation time. Obviously it could wait until asked, but it should also be more efficient.
Sure, DST rules change and a date library can't predict the future, but the alternative is to put an arbitrary upper limit on localized dates.
Everybody's already commented on the problems with always-changing DSTs. But I can accept the premise that we just pretend the currently known rules will apply forever.
To get your DST information, the first thing to do is to calculate the year/month/day for your future date (if it isn't in that form already). Then you look up your time zone and pull out the variation against UTC, the DST on/off rule and offset. There could be several different rules depending on which year, you want to be sure to grab the right one for your "target" year. For reasons explained below, it may be handy to also be aware of the rules for the preceding year.
The on/off rules will have a funny spec like "Oct lastSun": That means the switch occurs in the night of the last Sunday in October.
What you need to do is gather up all of these tersely formatted "rules" and develop a bit of code for each to determine the last date indicated by that rule. It's currently December, so given a couple of rules like "Mar lastSun" and "Oct lastSun" for my time zone, those dates would be March 29, 2009 and October 25, 2009. Which of these dates is more recent? October. October is associated with an "off", so we must currently have NO DST.
You can calculate the DST on/off dates for the current (i.e. target) year regardless of whether the target date is before or after those dates; if the on/off date is in the future of your target date, then simply do the rule calculation again for the previous year. Note that the rules may have changed during the interval, so be sure to apply the correct one for the year you're looking at.
Worst case for this calculation is, you have to repeat your two rule calculations for the previous year. But there's no searching going on otherwise, so it's strictly O(1).
I found a Local/DST/Tz calculator here: http://home-4.tiscali.nl/~t876506/WhatDay.html and as it's a JavaScript applet you should be able to simply crib the code. It doesn't handle all rules, though, so you will need to add some code for the remaining rules.
Update: I just noticed you have an hour and minute in your time as well. That complicates matters just a little. If your date is not on a "switch" date then the instructions I gave above will do you fine. Otherwise, you need to consider the time. I guess the cleanest thing to do would be to include the time in your determination of "most recent". I.e. if your target time is 00:30 UTC and switch time for the given zone is 01:00, then the target year's switch time is still in the future and you have to use the previous year's switch time instead. For practical purposes, this will mean that the "other" switch time was the most recent, and its on/off status applies.
Your number one problem is daylight savings rules that are set by the local authorities. The latter can pass almost any law at any time and therefore change the rules in a way you can't possibly predict.
As far as I know DST changes that are known start and end on a fixed day each year (first weekend in april, last weekend in october, stuff like that). So you could ese the Doomsday Algorithm to find the days of the week for the given year and calculate the conversion dates from that. Then you can determine if DST is in effect in source and/or destination locale. The converion itself is simply a matter of adding and/or subtracting an hour to compensate for DST and then factor in the timezone difference.
Hmm, as I see the problematic point is to determine weekday for a given day, far in the future.
For that, I suggest something like that:
after every 400 years, the complete system turns around, so first divide the number of years with 400, take the integral part. In 400 years, there are 99 leap years and 301 simple ones. If an arbitrary day is Monday, then the same day 400 years later will be 301+2x99 = 499 (mod 7) ---> Monday+2 ---> Wednesday. So you have to say something like that:
wday = (ref_day + 2 * (int)((target_year - ref_year) / 400)) mod 7
then you can do further optimizations, but I guess you can go year-by-year, that will do it. At most you make 399 simple operations, if (leap_year) then ++ else +=2, mod 7.
After you have the weekday for Jan 1 that year, you can calculate DST switching dates, as Carl Smotricz has written.