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Discussion of the The Chronos Date/Time Library, the Smalltalk programming language, and new developments of significance in science and technology.

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2007-02-02

Why don’t calendars do Time Zones?

Jim Phelps asks (2006-01-18) "Why don’t calendars do Time Zones?"

He laments "My laptop and my palm both understand Time Zones. I understand Time Zones. Why don’t my calendar applications (Oracle’s Calendar and the Palm Calendar in the handheld) understand Time Zones?"

Jim has the following "simple" requests:


  1. When I create an appointment I should be able to mark the Time Zone for the appointment.

  2. I should also be able to make appointments that are Time Zone neutral - not tied to a Time Zone.

  3. My clients (Palm and Desktop and Web) should all understand Time Zones and should be able to shift the alarms to compensate for my changes in Time Zone.

Here's how Jim would use such functionality, were it only available:

  • If I get an invite to join a conference call at 11AM EST, I could just enter that time without adjusting for my local time zone. Same with UTC time.

  • If I changed time zones (say fly from Madison to San Francisco), my alarms would change to be appropriate. As it is now, if I fly to California, I need to keep track of the fact that the 10AM meeting is really a[t] 10AM Central time so I need to set an alarm 2 hours earlier. But I can’t do a global change because some things might be dinners in California so they need to stay on Pacific time.

Of course, Jim's not the only one whose Use Cases aren't well satisfied by most current applications and/or date time libraries. For example, a few years ago, some date/time Use Cases were posted on Zope.org, because Zope's date/time functionality was seen as unsatisfactory. And on BoingBoing, there was the rant "Extended iCal rant from a timezone warrior," whose basic complaint resembles Jim's. There was also recently a discussion on Bugzilla where some of the same issues and concerns were raised.

Here are the key requirements that can be abstracted from the aforementioned discussions/rants:
  • It must be possible to associate each point-in-time value with a time zone that may or may not be the same as the time zone associated with other point-in-time values.
  • It must be possible to specify a point-in-time without reference or association to any particular time zone. (Examples: The point-in-time that specifies the first moment of 2008 (for any and all time zones,) the annual date on which a person's birthday is celebrated, the list of non-business days of the US Federal government, the date on which a person must have been born in order to legally purchase alcohol today.)
  • Point-in-time values must either know, or be able to derive, both their local time and their Universal Time.
  • It must be possible to convert point-in-time values from one time zone to another.
  • The same point-in-time value must be able to act as though it is associated with ("local to") a different time zone in different contexts (e.g., the possibly-different local time zone of different users at the same time, and also the local time zone of the same user at different times, even when the user's local time zone changes as he travels from one location to another.)
  • Point-in-time values should provide both sufficient sub-second resolution and sufficient range (from earliest to latest representable value.)
  • Point-in-time values should provide appropriate granularities--timestamps should span a reasonably small sub-second duration, but dates should span a calendar day (and not be sub-second-resolution timestamps that happen to specify the first moment of the day.)
  • Point-in-time values must be immutable so that changes to their value don't invalidate the logic that uses them (e.g., they must be useable as dictionary keys.)
  • Adding any combination of some number of days, months and years to a point-in-time value should result in a new point-in-time value that number of days/months/years distant--but having the same civil time of day, regardless of any DST transitions that may have occurred.
  • It must be possible specify partial/recurring dates and times (e.g., "every/any day at 5pm," "the last Sunday of October every/any year.")


I was especially interested in what one of the Zope developers had to say in response to the Zope Use Cases posted by the general public (spelling and punctuation as in the original):

"I find all of that very allarming.

Do people want the datetime object solve all this use cases out of the box? I hope that they are only samples of applications and that people agree that they will have to do a little work by theimself.

It is already difficult to find a straightforward interface for basic operations, if we wait to have an universal datetime type, we will never have it before Python 48.12b1 release.

So please, go for a minimal API, and let people play with the type and discover the better way to do elaborated stuff.

By the way from the tz database README file at: http://www.twinsun.com/tz/tz-link.htm you can't guess a time zone from a time-zone offset and a daylight saving flag.

So I think that a full and cross-platform timezone support is just a wonderful dream. But if you can prove me that I'm false, with an actual implementation ..."


I'm quite sure that many programmers would have a similar reaction: They'd be highly alarmed. Well, tough. People don't care how you solve their problems, they just want them solved. A solution that doesn't work right can be worse (or no better than) not having any solution at all.

We've had "a minimal API" for date/time computations available in just about every programming language and operating system for decades now--but the requests, complaints and rants shown above are from recent times, not from decades past. It's almost universally true that programming languages, operating systems and applications either don't handle dates/times at all, or don't do so correctly and consistently, and don't satisfy most of the requirements discussed above.

"Do the simplest thing that could possibly work" is not a license to provide incomplete and/or incorrect functionality. Real use cases that are commonly encountered are not being satisfied. The first step to a cure is to admit there's a problem.

Dates and times are not simple. Not at all. They're a relatively hard problem. The truth is, dates and times are too hard for most programmers to handle correctly on their own--which is precisely why they don't do it, and instead attempt to justify their failure by misapplying design principles. And that's why the "provide the simple stuff, and let the application coders solve the hard problems" approach won't work. It doesn't work in other domains, either--not in the past, not now, and probably not ever.

The "let them do it themselves" approach is about as wise as letting each application programmer code his own number classes, his own collection classes, his own filesystem, his own garbage collection engine, his own virtual memory subsystem, his own graphics library and GUI framework, his own web/application server--and in his spare time, develop his own programming language and compiler. If you really feel that a date/time library that actually gets the job done would be just too complex, then to be consistent, you would have to raise the same objection to all the other tools that programmers don't do themselves anymore (because the problems are hard, and most programmers will take unwise shortcuts, or make serious errors, if they have to do it themselves.)

As time has gone by, we've gone from machine language, to assembly language, to procedural languages, to functional languages, to object-oriented languages. We've gone from no operating system, to reusable, statically-linked code libraries, to dynamically-linked code libraries, to run time systems, to basic operating systems, to operating systems with virtual memory, to operating systems with GUIs. We've gone from punched cards to Microsoft Word, and from the abacus to Microsoft Excel.

Does anyone see a pattern here? Yes, you have to walk before you can run. But we've been crawling around trying to avoid actually dealing with dates and times in all their glory for far too long now. It's time to advance to the next level. So, to the programmers of the world, I say: Admit it. Admit that you're not satisfying the real date/time use cases.

As for an actual implementation of a date/time library that satisfies the requirements we've been discussing, it happens that there is at least one: The Chronos Date/Time Library. Of course, it's a date/time library, not a calendaring/scheduling application, so it would be more accurate to say it makes it much easier to write applications with just about all of the requested features. Yes, applications do need to take some responsibility for their own particular use cases.

What are the features of Chronos that satisfy (or greatly help to satisfy) the requirements discussed above? They are as follows:

  1. Chronos uses the Olson Time Zone Database, so it knows about all the world's time zones--including historical information. For example, Chronos knows, and is able to correctly handle, all the following facts:
    • Most (but not all!) locations in North America have been switching from standard time to daylight saving time on the first Sunday of April each year, from 1987 through 2006--but starting in 2007, most North American locations will switch from standard time to daylight saving time on the second Sunday of March.
    • All the countries in the European Union modernly transition to and from daylight saving time at the same moment in Universal Time--unlike the case in North America, where all zones with the same offset transition to/from daylight saving time at the same local time, but at a different moment in Universal Time.
    • All of India uses the same time zone offset, does not observe daylight saving time, and uses an offset of 5.5 hours east of Universal Time.
    • In Australia, South America, New Zealand and Africa, daylight saving time, if it is observed at all, is observed during the Southern Hemisphere's summer (for example, from September to April.)
    • During World War II, several countries were on "daylight saving time" all year round for several years in a row (in the US, this was called "War Time," and was abbreviated as "EWT," "CWT," "MWT" and "PWT.")
    • For the years 1921 and 1922 only, Moscow started the year with an offset of 3 hours, transitioned to an offset of 4 hours on 1920-02-24T23:00 and 1921-02-24T23:00, transitioned to an offset of 5 hours on the 79th day of the year (different dates and day-of-the-week each year,) transitioned back to an offset of 4 hours on the 244th day of the year, and then transitioned back to an offset of 3 hours on the 274th day of the year (so there were 4 offset transitions and 3 different offsets each year.)

  2. In addition to the predefined time zones that come from the information provided by the Olson Time Zone Database, Chronos permits you to define your own time zones--which can be as simple as "-5 hours asTimezone" or as complex as any of the cases described in the preceding bullet item. Or you can easily construct a Chronos time zone from either a POSIX time zone rule literal or from the information in the Windows registry--and it will behave identically to its UNIX or Windows counterpart.

  3. The Chronos Time Zone Repository (which is generated from the Olson Time Zone Database) is deployed separately and independently from the Chronos codebase. Either can be changed without any need to redeploy the other--and even already-running applications can see and use a newly-deployed version of the time zone repository. And both Chronos and the Chronos Time Zone Repository are available for Windows, Mac, Unix and Linux--so your applications will not only run unchanged in all those environments, but will exhibit identical date/time behavior.

  4. Chronos point-in-time values can be associated with ("bound to" in Chronos terminology) a particular time zone. Or, they can be bound to a proxy time zone that dynamically refers to a different time zone at different times or in different contexts (so that the associated point-in-time's local time changes dynamically whenever the proxy time zone is changed to have a different time zone as its referent.) Or, Chronos point-in-time values can be bound to a special time zone that Chronos refers to as "nominal time." Nominal time represents the concept of "any time zone," or "local time in some unspecified context, or in any and all contexts simultaneously."

  5. Chronos point-in-time values that are bound to a specific time zone or to a proxy time zone (and so aren't bound to nominal time) are said to be invariant to Universal Time. Chronos point-in-time values that are bound to nominal time are said to be invariant to nominal time. Point-in-time values that are invariant to Universal Time keep the moment in time that they represent in Unversal Time as their semantic invariant. Point-in-time values that are invariant to nominal time keep the moment in time that they represent in nominal time (their "local time") as their semantic invariant. In other words, two points-in-time that are invariant to Universal Time are equal when they represent the same moment in Universal Time, regardless of the moment they nominally specify in their local time zone (the one to which they are bound.) But a point-in-time that is invariant to nominal time is equal to any other point-in-time that specifies the same nominal (local) time. So the following expressions both evaluate to true:
    • (Timepoint 
      year: 2007 month: 2 day: 3
      hour: 10 minute: 0 second: 0
      timeZone: -8 hours)
      = (Timepoint
      year: 2007 month: 2 day: 3
      hour: 13 minute: 0 second: 0
      timeZone: -5 hours)
      "10 am in California is the same moment
      in Universal Time as is 1pm in New York"
    • (Timepoint 
      year: 2007 month: 2 day: 3
      hour: 13 minute: 0 second: 0
      timeZone: Timezone nominal)
      = (Timepoint
      year: 2007 month: 2 day: 3
      hour: 13 minute: 0 second: 0
      timeZone: -5 hours)
      "1 pm nominal time is 1pm local time,
      regardless of time zone"

  6. Chronos point-in-time values that are invariant to Universal-Time internally store their value in Universal Time, but report their year, month, day, hour, minute and second according to the equivalent local time in the time zone with which they are associated. Also, the local time values for their year, month and day-of-month are cached (lazily computed when needed--it's an expensive computation.)

  7. Chronos point-in-time values can easily (and efficiently) be converted from any time zone to any other:
    (Timepoint 
    year: 2007 month: 2 day: 3
    hour: 13 minute: 0 second: 0
    timeZone: 'Pacific/Honolulu') >> #'Asia/Katmandu'
    results in 2007-02-04T04:45:00+05:45 (and yes, Asia/Katmandu actually does have an offset of 5 hours 45 minutes.)

  8. Chronos point-in-time values have no limits (other than available memory on your computer) with respect to the range of representable dates--a feature that derives mostly from the fact that Chronos is implemened in Smalltalk--although, even if the implementation language had limited the design to using nothing but 32-bit signed integers, the way Chronos internally represents point-in-time values would still have supported a range of +/- 6 million years for timestamps (and +/- 2 billion years for dates.) Chronos also provides a special value that represents the infinite past, and another that represents the infinite future.

  9. Chronos point-in-time values are actually time intervals, as explained in the Chronos blog entry Chronos 101: "Points" in Time. Chronos points-in-time that are meant to represent dates have a temporal extent (and hence resolution) of one calendar day. Chronos points-in-time that are meant to be used as timestamps have a temporal extent (and hence resolution) of one nanosecond. In addition to these two common cases (one optimized for dates, the other for timestamps,) Chronos provides general Timeperiod values, which can start at any point in time, and can have any temporal extent/duration.

  10. Chronos point-in-time, durational and time interval values are all immutable.

  11. Chronos handles date/time arithmetic correctly, satisfying both civil and scientific/technical use cases. Adding a day results in the same civil time of day on the next calendar day.

  12. Chronos provides a variety of classes for representing partial/recurring dates--including the time-of-day without any associated date, and the month/day without any specified year. You can even construct a value that specifies "the first Tuesday of November, every fourth year modulo 4."

And there's yet more. If you're interested, you can peruse the Chronos web site, browse the Chronos blog, or read the "Chronos 101" tutorials, of which the following have been published so far:

So, to answer Jim's original question, calendar applications don't understand time zones because most people, including software developers, programming language library creators and programming language designers don't understand time zones. Or in some cases, they understand them well enough, but decide (for whatever reason) that full and correct time zone behavior is more work than they want to take on. Fortunately, that unwillingness to tackle the date/time domain comprehensively is not universal.


2 comments:

Anonymous said...

Hi,
When i am sending the first century date from my client to linux server(for e.g for 1st Jan 1975 i am sending 01/01/75, as we are knowing 75 means it is 1975 but the linux server treats it as 0075(inserted two zeros) and return the response which leads to test failure. Can anybody know how to solve this problem, so that linux server not insert two zeros and return only year 75 in a response?


Thanks for your help in advance..

Thanks,
Sangram.

Alan Lovejoy said...

You don't provide sufficient information to provide anything more than general advice:

Never use 2-digit years when exchanging dates between programs. Programs should always use 4-digit years internally, and when exchanging dates with each other. 2-digit years should only be used when showing dates to users, or parsing dates entered by users.