I am writing an app that communicates with some web services. When a variable is updated on the server, it will return a unix timestamp to tell me when it was updated. I can then compare it using:
[[NSDate date]timeIntervalSince1970]
to keep things in sync.
The system time on the iPhone is correct. But unix time seems to be 1hr behind the server(and system time as shown on the clock). Why is this happening? Do I need to set a time zone?
timeIntervalSince1970 is defined to return:
The interval between the receiver and the reference date, 1 January
1970, GMT.
Is it possible you're comparing to a time that is defined to be from 1 January 1970 in some other timezone? If so, the easiest thing to do is to use timeIntervalSinceDate: rather than timeIntervalSince1970, possibly having used NSDate's dateByAddingTimeInterval: to move from the reference date in GMT to the reference date in your ideal time zone.
You can use NSTimeZone's secondsFromGMTForDate: property if you don't want to hard code things.
As it can be understood from your question, the problem is in the timestamp that returned by server. So the real question is if you have an access to that server. Or do you know in what timezone it is. If first, update time on server. Second - calculate difference between timezones (you say it's a hour) and add it to your NSDate instance.
You also may use NSCalendar as it have -(void)setTimeZone:(NSTimeZone *)timezone; method.(thought it may be overhead for your needs)
Related
Some events don't take place at any specific time and instead are meant to be valid for the whole day irrespective of the time zone the user is at.
For the sake of argument, let's say a system sitting on a server (up in the cloud) runs a job at 5 am and imports data from a different system between this run and the last (24 hours ago). The actual user sitting at his desk doesn't know when the job runs, the user only knows that they go to sleep at night, the server crunches all the entries for the day.
The next morning the user wants to see all the entries from yesterday (what ever the job produced) and they go to the app, pull up a calendar input selector and they pick the 5/26/2022 (today being 5/27/2022).
Assuming the developers followed best practices, the client will transform the date into it's UTC version and send it up through an API. Chances are, depending on where the user is located and the server is, there might be a mismatch.
I could send the date up without it being UTC or I could send a UTC date and try to adjust it back to local time so that I could then compare with the date on record (that exists without an actual time zone).
What I am asking is:
What's the more conventional answer to this particular problem?
Is the idea of a date without time or time zone just ridiculous?
Use UNIX Time. It will give you a timestamp that is universal no matter what timezone the user is in. You can then convert it into whatever timezone you want to.
The concern you describe is well solved/addressed by the ISO 8601 dates/time presentation protocol.
All modern software can read/write dates in ISO 8601.
In Unix machines, the correct command is date with option -I
-I[FMT], --iso-8601[=FMT]
output date/time in ISO 8601 format. FMT='date' for date
only (the default), 'hours', 'minutes', 'seconds', or 'ns'
for date and time to the indicated precision. Example:
2006-08-14T02:34:56-06:00
At 6:00am Sydney time, Rory Allan clicks the foofoo button in their browser.
This inserts into the foofoo table with an 8:00pm UTC timestamp in the database of my server.
A lot of people are excited about this foofoo button, and they wanted to see what time of the day Sydney time Rory clicked it.
As I stare at my screen in California and view the status of the click, I want to see that he clicked it at 6:00am, not 1:00pm, which is what I will see if I pull the UTC time from the server and let my browser convert it.
You see, a lot of different people click foofoo, such as Phillip Herman in Germany, or Ivan Efimiov in Russia. And all we care about is the relative time of the day they clicked it, in the location they clicked it, regardless of the viewing location.
I don't know the best way to do this. Do I take the local timestamp and convert it to a string, storing it in addition to the real UTC timestamp? Or is this a common problem with a common resolution that I haven't found? I'm guessing / hoping the latter.
This isn't Language specific. These dates have a long journey:
Unix timestamp > Python date > JSON > Node > Mongo > Node > Browser
Ok, clearly 8:00pm UTC is not enough information. But what do you really want to know?
Is 06:00 enough information?
Look for time-only data types, preferably those that reflect time-of-day, rather than elapsed-time. These are often called LocalTime, TimeOfDay, etc.
Or, use strings in HH:MM format (on a 24-hour clock)
Or record the total number of minutes as an integer (60 * HH) + MM.
Or if you need a higher precision, then use seconds, or milliseconds, or microseconds, etc...
Is 06:00 Australia/Sydney enough information?
Store the time as mentioned earlier and store the time zone name in a separate string.
But be careful, because without a date you don't know if UTC+10 or UTC+11 was in effect.
Is 06:00+10:00 enough information?
You could look for a time with time zone data type, such as the one that exists in PostgreSQL, though even the PostgreSQL docs strongly discourage using this type.
Instead, store the time and offset as separate components.
Be careful, because you'd not necessarily know if this data was from Sydney. There are other time zones with UTC+10 at parts of the year too.
Assuming you have the date, such as today, then:
Is 2017-05-31T06:00 enough information?
Store the date and time together in a component that is sometimes called DateTime or LocalDateTime in various languages - but be careful that it is not bound to any specific time zone. Use DateTimeKind.Unspecified in .NET, or "naive" DateTime's in Python, etc.
For MongoDB and others that don't have such a type, use a string in ISO8601 format.
Be careful, because this time could come from anywhere in the world. Nothing here relates it to a specific point in time.
Is 2017-05-31T06:00+10:00 enough information?
Some languages and databases have a DateTimeOffset or OffsetDateTime type for this purpose.
If not, you can store a "Unix Timestamp" and also store the offset from UTC separately.
Or you can just store a string in ISO8601 format with the offset included - just be very careful with regard to comparisons / sorting.
Is 2017-05-31T06:00 Australia/Sydney enough information?
Store separately the date+time component from the time zone.
Consider that a time may be ambiguous during a DST fall-back transition.
Is 2017-05-31T06:00+10:00 Australia/Sydney enough information?
Here we have everything we might possibly need. Use a ZonedDateTime in Java/Joda-Time/Noda-Time, or an "aware" DateTime in Python (pytz, dateutils, etc.), or similar types when they exist in your platform.
Watch out for timestamp with time zone, as you might expect it to store the time zone and it typically does not (despite the name).
If not available, then consider storing a Unix timestamp and separate the time zone name as a string.
As you can tell - there are a LOT of options, and it really depends on exact use case and features available in each language/platform. You'll find more details if you search/ask for each one separately.
tl;dr
Exchange date-time values in UTC as strings in standard ISO 8601 format.
Work in UTC
General rule in date-time handling is to think in UTC, work in UTC, log in UTC, share in UTC, and store in UTC. But present in zoned time for the user.
By think in UTC, I mean every programmer needs to learn to stop their parochial thinking about their own particular home time zone. Translating back-and-forth to your own zone to UTC to other zones will drive a person nuts. Think of UTC as The One True Time®. All other zones and offsets are mere variations.
This strategy is much like internationalization. The programmer uses key strings in her own human language to look up string values from the localization tool to present a value (piece of text) in the human language preferred by her user. In date-time handling, the programmer works in UTC but applies a zone preferred by her user for presenting text in the user-interface.
Moment in UTC
The basic Java class for this Instant. 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).
So when Rory in Sydney clicks his button, we record an Instant object.
Instant instant = Instant.now() ;
instant.toString(): 2017-06-01T09:24:54.435Z
Zoned
To present that moment to Rory in his own time zone, we apply a ZoneId to get a ZonedDateTime object.
Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).
ZoneId z = ZoneId.of( "Australia/Sydney" ) ;
ZonedDateTime zdt = instant.atZone( z );
zdt.toString(): 2017-06-01T19:24:54.435+10:00[Australia/Sydney]
Now we have two objects, instant & zdt, that both refer to the same simultaneous moment on the timeline. The only difference is wall-clock time.
If the user in California wants to see the moment of Rory's button click according to Sydney time, then we already have that solution seen above. If not, if the California user wants to see the moment of Rory's button click in her own California clock, then read on.
We can adjust into yet another region’s wall-clock time by applying another time zone.
ZoneId zAmericaLosAngeles = ZoneId.of( "America/Los_Angeles" );
ZonedDateTime zdtAmericaLosAngeles = instant.atZone( zAmericaLosAngeles );
zdtAmericaLosAngeles.toString(): 2017-06-01T02:24:54.435-07:00[America/Los_Angeles]
Now we have three objects that all represent the same simultaneous moment: instant, zdt, and zdtAmericaLosAngeles. One moment, three wall-clock times.
See this above code run live at IdeOne.com.
Time-of-day
If you literally meant you want the time-of-day only, without the date, you can extract a LocalTime object from those objects above.
But think twice about doing this, as presenting a time-only without date and zone can lead to ambiguity and misunderstanding.
LocalTime lt = zdt.toLocalTime();
all we care about is the relative time of the day they clicked it, in the location they clicked it, regardless of the viewing location
If you are really really really sure that is what you want, then combine my advice above. (But I doubt this is a wise way to go.)
LocalTime lt = LocalTime.now( ZoneId.of( "Australia/Sydney" ) ) ; // Current time-of-day in Sydney.
Library for date-time
We have been using the modern java.time classes in examples above. They are exceptional – I mean than literally. They are virtually unique. Virtually all other platforms have terrible support for date-time work. The predecessor to java.time was the Joda-Time project which was ported to .Net platform as Noda Time. Other than java.time & Noda Time, I know of no other decent library on other platforms.
ISO 8601
The ISO 8601 standard defines many sensible practical formats for textual representation of date-time values.
The java.time classes use the standard formats by default when generating & parsing strings. You have been viewing those ISO 8601 formats in examples above. Except for ZonedDateTime which wisely extends the standard by appending the name of the time zone in square brackets.
The T in the middle separates the date portion from the time-of-day portion.
For UTC, the Z on the end is short for Zulu and means UTC.
For offset-from-UTC, you see a plus/minus number of hours and minutes ahead of or behind UTC. A time zone is history of past, present, and future offsets in use by a particular region.
Database
How databases handle date-times varies widely, though poor support is most common. A database driver, such as JDBC drivers, add another layer of behavior. So no way to succinctly address that here. And this topic is already asked and answered in many other pages in Stack Overflow.
If your database lacks serious date-time support, you may be better off storing the ISO 8601 strings with the Z on the end. These values when sorted alphabetical are also in chronological order.
I'm using GWT on the client (browser) and Joda Time on the server. I'd like to perform some DB lookups bounded by the day (i.e. 00:00:00 until 23:59:59) that a request comes in, with the time boundaries based on the user's (i.e. browser) timezone.
So I have the GWT code do a new java.util.Date() to get the time of the request, and send that to the server. Then I use Joda Time like so:
new DateTime(clientDate).toDateMidnight().toDateTime()
The trouble of course is that toDateMidnight(), in the absence of a specified TimeZone, will use the system's (i.e. the server's) TimeZone. I've been trying to find a simple way to pass the TimeZone from the browser to the server without much luck. In GWT I can get the GMT offset with:
DateTimeFormat.getFormat("Z").fmt(new Date())
which results in something like "-0400". But Joda Time's DateTimeZone.forID() wants strings formatted like "America/New_York", or an integer argument of hours and minutes. Of course I can parse "-0400" into -4 hours and 0 minutes, but I'm wondering if there is not a more straightforward way of doing this.
You could use java.util.Date's getTimezoneOffset() method. It's deprecated, but that's pretty usual for Date handling in GWT currently.
And AFAIR, you can specify something similar to "UTC+4" in Joda time.
Update: I looked it up, and it's "+04:00". Or use DateTimeZone.forOffsetHours() or even forOffsetMillis().
Gwittir (http://www.gwtsite.com) is a library for GWT that includes many cool utilities, like databinding, animation, reflection, and more. However, there are some other interesting goodies as well like the new Joda Time integration. If you have ever been frustrated by GWT’s lack of java.util.Calendar support, you’ll love this, as it makes it easy to do date manipulations in your applications.
otherwise, there are other ways to get timezone offset with + & -.
import java.util.TimeZone;
use: TimeZone.getDefault().getRawOffset()
this function will return the offset time in millisecond about your phone seeting. For Example, GMT-04:00 is equals to (-4)*60*60*1000 = -14400000.
After some operations to get the number which you want.
I have a similar but slightly different problem I think.
I actually need to store the clients timezone on the server, so that I can send out messages about dates stored in their calendar.
The dates are stored in UTC time in google app engine and of course I can store the current Timezone offset when creating the appointment. The problem comes when for instance I want to send out a summary email with a list of upcoming appointments in it. These appointments need to be offset with the correct Timezone adjustments for the client (Im happy to assume that they are still in the same timezone as when they created the appointment).
The real problem comes with Daylight Savings adjustments, so for instance I might have appointments stored for Saturday 30th October 2010 at 1pm (BST[GMT+60]) and Monday 1st November 2010 at 1pm (GMT).
So as you can imagine, I cant just use the current timezone offset (BST) as that would mean that the appointment on Monday 1st November would be listed as 2pm rather than 1pm (GMT+60)
It occurs to me that the best way to deal with this is just to store the timezone offset with each appointment individually, but I feel it would be much better to be able to determine the original timezone correctly in the first place, then just let java do the correct adjustments.
From my iPhone app I insert some data in my sqlite database that include a date using the CURRENT_TIMESTAMP default value. Everything works great except for the fact that the time is 1 hour behind the time it should be. And that happens both on the device and the Simulator.
Any sqlite settings (like current time) i can access somewhere?
If you want to see the code I am using for that you can take a look at my answer to this post: sqlite datetime data type with iphone NSdate?
It sounds as though there might be a day light savings time zone inconsistency between SQLite and the time zone that NSDateFormatter is using. It looks as though SQLite stores the datetime in GMT. Therefore you'll need to make sure that NSDateFormatter is also set to GMT. It appears as though you can do this with:
gmtFormatter.timeZone = [NSTimeZone timeZoneWithAbbreviation:#"GMT"];
[gmtFormatter setDateFormat:#"yyyy-MM-dd HH:mm:ss"]
Once you have your NSDate instance I'd expect that you could for example get a string version of it showing the correct local time using a NSDateFormatter instance that is set with the local/system time zone.
Note: I haven't actually tried this.
You might want to consider using Core Data which is now available in O/S 3.0 - this handles things like this automatically for you without you having to specify timezones etc - it will just use the user's defaults and keep everything consistent.
There are many other advantages - but this seems like a relevant one.
I guess you better implement teabot's suggestion. The Problem comes when one timezone has wintertime and one doesnt...
I have just had to fix this problem also, and teapot's answer is the correct one.
The timestamp created by sqlite is in GMT, so when you read it from the database you need to tell your date formatter to read it in GMT also. The resulting NSDate will be a date correct in whatever timezone you are in.
The way you say you have corrected it, by adding 1 hour, will eventually fail on you. Anyone using the app in the GMT timezone will see all times +1 hour. Anyone in other timezones will see a completely different time offset.
(I know this is years old, but it has just helped me).
Well, since I couldn't find where this 1 hour difference comes from, I used the following method to fix the date. It's not a proper solution but it does exactly what i need. So in case someone else runs into the same trouble as me.. here's my fix:
NSDate *date = [[NSDate alloc] initWithTimeInterval:3600 sinceDate:[formatter dateFromString:score.datetime]];
which is just adding a one hour delay to the datetime, therefore making it correct.
I'm developing an international software that act as a simple project management software and I'm facing a problem. This problem is about date/hour and time zone.
When a message is sent from one time zone to another time zone I can store the UTC (GMT) time in my database and then have it displayed differently according to the user's time zone. But this can't be done when I only work with date.
If I say a task is due to the 21st of March. Should I consider that this date can be 20 or 22 in some other countries ? What are your advices on this problem ?
Let's say a user in New York sets a due date for a project as "anytime on Monday 26 January". That means "anytime from 0600 Monday 26 January to 0600 Tuesday 27 January" in Brussels and "anytime from 2000 Sunday 25 January to 2000 Monday 26 January" in L.A.
So completing the task at 2100 on Monday 26 is fine in Brussels and N.Y., but too late in L.A.
One possible work around is never just work with the date. If the time is not specified, either set it for 0000 hrs or 2400 hrs on the date specified in the timezone of the user.
The users may have to deal with strange due dates/times, but speaking as someone who used to work internationally, it kinda goes with the territory.
You won't be able to achieve what you are trying to do without storing the exact time. You simply don't have enough information.
When you don't have a time, assume that the time is the end of business in the main locale for the application, then translate that time as you would any other time. An alternative would be assuming end of the business day in local time and adjust that to UTC. Everyone using the application would need to understand whatever default time assumption you make when the time is not specified. Coordinating to the main office may be best in a large enterprise whereas coordinating to local time may be best in highly decentralized environments where the local context is equally important.
If you aren't storing the minutes and seconds you have to assume that the date being entered is the desired date and not to any adjustments for GMT. Just put it in the database as is. The people on the west coast will have to assume that the due date is the same regardless of where you are in the world. If you want to adjust for time zones, you'll have to collect more information, like hour, minutes, and seconds.
The easiest solution would be just to display as the same date for everyone. The deadline would then effectively be midnight in the latest timezone.
Otherwise, decide what the default time of the deadline should be in the timezone the task was created in, e.g. 21st March 17:00 EST or 22nd March 00:00 EST and display that in the local timezone. The timezone difference will then push it into the previous day or next day accordingly for the viewer.
SQL 2008 allows for a Date datatype that does not have any time value associated with it. That allows someone to say I need this done by this Date, but I don't care if it is +/- several hours. If the date selected is 1/1/2009 but it happens on 1/2/2009 at 2AM their time, they probably don't care.
When the user needs something done by a specific date and time, like close of business on 1/1/2009 then you need to store it in a DateTime as UTC and convert it to local time client-side.
This will take much of the complexity out of indicating when something is completed, it'll either be completed near a specific day or by a specific time.
If you have a single instance of a DB, I would store all dates in the datetime timestamp of your DB server. If you are timestamping rows, consider GetDate() in T-SQL or as default value of the timestamped date column. Then you have your single reference point for all times. Consider UTC format there.
Then, all clients accessing the date do their own conversion into "local time" , which can be interpreted by things like : user preferences, date time stamp on client computer, etc.
Without knowing more, it hard to say exactly what the resolution is.
Your solution depends on your application and requirements.
I'd first store UTC + offset in your data structures, so it's easy to display for any timezone.
Most likely if a task or meeting is due at 12pm on 21/March in London then it will occur at 2130 on 21/March in Adelaide (+0930), but that is an application requirement not any sinister timezone related standard.
If you want the ultimate in flexibility, add a flag that can make the even due simultaneously in every timezone or at the same time no matter where you are (staggered) and show the event accordingly.
You might want to store the date in a from that is timezone aware. This will help you in your calculations. SQL Server 2008 for instance supports a datetimeoffset that does precisely this. Alternatively if you're using SQL 2005 with a bit of effort you can write your own SQL CLR data type to support this.