G'day All
If you create a date item in the plist editor of Xcode or Apple's standalone plist editor you get something of the form <date>2010-05-29T10:30:00Z</date> which is a nice well formed ISO date at UTC (indicated by the "Z"). Because I'm in timezone UTC +10 when that's read into my app & then displayed I get 8:30 PM out, still good. However if that is a time in my timezone it should be <date>2010-05-29T10:30:00+10</date> (replacing "Z" with my timezone offset). All of my attempts at reading such dates into my iPhone app have had the plist rejected as if it is malformed & editing a plist with such a date in Apple's editors changed the "+10" to "Z" without adjusting the time.
Do others think I'm correct in thinking this is a bug in either plist or Xcode? My feeling is that the implementation of ISO date & time in plist is incomplete.
Cheers, Pedro :)
It's not a bug in either. In CoreFoundation (and Foundation), all dates are represented in Zulu time, which is why they are serialized that way. The date is then formatted for display based on the timezone of the device that wishes to display it. Although this is an ISO date string, the only valid time zone for CoreFoundation/Foundation is Zulu time.
If, for some reason, you need to track the time zone that any given date was generated in, you should track this as a separate property. If you need to write an XML property list from somewhere else, you must first convert the date to zulu and then write it out (although the documentation clearly specifies that these keys are for debugging aids/readability only and may change in the future). This makes plists a decent way to serialize data between two Cocoa/CoreFoundation applications, but a less suitable way of serializing data between a Cocoa/CoreFoundation app and some other application.
Related
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.
Im using cfspreadsheet to read data from spreadsheets inside one of my applications. I've had a great deal of difficulty dealing with date columns. If I format the cell as date english (NZ) it displays right in the spreadsheet, but when I try to upload it switched the day and month. But If I change the format to a custom "dd/mm/yyyy" format it will upload without a problem.
Why would using the default date formats within the spreadsheet mess up the format when a custom one doesn't? Is there a work around?
I was using the Date Type "*14/03/2001".
The * means that it will:
respond to changes in regional date and time settings that are specified for the operating system.
So it must have been getting turned about by java or CF somewhere along the way. I changed it to the same date mask without the asterisk and the problem stopped occurring.
I've read many a post here re: GWT date handling.
One in particular that struck a cord with me was this one
Sending a date and timezone from GAE server to GWT client
Anyhow, there's a need on a project I'm working on to be able to display days, hours, minute intervals as labels in a grid. My team has adopted an approach where all date/time instances are passed the client from the server in ISO8601 String format. The server time zone is to be respected by the client. The biz use case is that all date/time instances are in "market time", so that any browser that visits the app will see and work with dates in the "market time" timezone which happens to be GMT-05:00 (if Daylight Savings in effect) or GMT-06:00 (if Standard Time in effect).
I have posted some source on Github, here:
https://github.com/fastnsilver/gwt-datehandling-example
Particularly...
https://github.com/fastnsilver/gwt-datehandling-example/blob/master/src/main/java/me/fns/gwt/datehandling/client/util/CSTimeUtil.java
and the GWTTestCase
https://github.com/fastnsilver/gwt-datehandling-example/blob/master/src/test/java/me/fns/gwt/datehandling/client/util/CSTimeUtilTestGwt.java
in the hopes that someone can stare at the utility (and test) we're employing for date handling and help us see what we're not seeing.
EDIT
The basic problem is that CSTimeUtil#hoursInDay(Date) is not being calculated correctly in Production mode for "transition days" This method is used by other methods (like CSTimeUtil#dateToHour(Date) and CSTimeUtil#labelsForDay(Date)).
I have deployed our application with the current implementation of CSTimeUtil and it appears to work, but not quite. I'm really confused by alternate test results when e.g., mvn gwt:test is run in GWT Mode or Production Mode on Windows where the OS timezone is set to various timezones other than U.S. GMT-05:00 or GMT-06:00.
Based on some hints from Andrei and some serious blood, sweat and tears, I figured this out on my own. I have updated the code in Github, so if you're curious please go have a look there.
The basics:
Make sure all Strings are ISO8601 (no millis) compliant when sent from server to client and vice versa
Use DateTimeFormat.getFormat("yyyy-MM-ddTHH:mm:ss.SZZZZ") to format and parse dates
Retreive GMT-prefixed time zone info from java.util.Date in "Market time" using DateTimeFormat(Date, TimeZone), where TimeZone param is set as TimeZone.createTimeZone(TZ_CONSTANTS_INSTANCE.americaChicago()) and time zone String retrieved by TimeZone.getISOTimeZoneString(Date)
Generating days, see generateDay(Date, int) or hours generateHour(Date, int), from a source date had to take into consideration that an increment or decrement coudl trigger a change in time zone offset if occurring on a "transition day".
If you time zone is fixed, why would you use a string to represent date/time? You can send a standard Java Date object to the client. If you want, you can even store all dates and times as Longs and pass Longs only. You also send the GWT's TimeZone Json string for your time zone (once per session). You can find it in the GWT - there is a file with strings for all time zones.
On a client you use DateTimeFormat with many predefined formats to display whatever you need: full date, month and date, date and time, etc. Just remember to create TimeZone object from this Json string and use it in DateTimeFormat.getFormat(...).format(Date, TimeZone).
With this approach you don't have to worry about DST changes (they are encoded in that Json string) and locales. You only pass simple Date or Long objects.
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.