The Timing of Time

Alan Lovejoy squeak-dev.sourcery at forum-mail.net
Fri Apr 21 17:46:22 UTC 2006


Hernan: "I did not say that Chronos had that problem, I just said what we
wanted to avoid, that's all... "
 
Ok, understood.
 
Hernan: "We have to add that functionality to Chalten because it is part of
the thesis that Maxi has to write."
 
Is the thesis requirement just that time zone and multi-calendar
functionality must be in the functional set, or also that Maxi must design
and implement the whole protocol stack, soup to nuts?
 
Whatever the case, a thesis requirement (or a desire to learn by doing) is
admittedly a valid reason to reinvent the wheel.  I've done the same myself.
 
Hernan: "I like the idea of using measure instead of duration because as
conceptual entities, they represent the same. The only difference is that
measures are more generic because they can be use as measure of time,
length, money, etc. So, I like the idea of using measure because when you
get used to it, you don't have to care anymore about all the different
classes that represent, in an abstract way, the same concept, like Duration,
Money, Length, etc. All those concept can be represented as Measures. But
hey, that's my idea and not everybody has to agree with me..."
 
I like Measures too--for the reasons you give.  But sometimes, you just need
simple numbers (or integers)--the Measure implementation may need them
internally, for example. In the case of Chronos, my policy was to strictly
conform to the ANSI Smalltalk Standard (which requires Durations to work the
way Chronos and Chronology implement them,) and to leave the implementation
of "Measures" to either some other time, or to some other developer. I
neither can nor should do everyting, and am actually looking forward to
collaborating with others for help with a) porting to other Smalltalk
environments,) b) getting low-level extensions implemented that Chronos can
use (in Squeak, the opportunities are in the area of locales and system time
zone discovery,) and c) implementing higher-level abstractions on top of
Chronos, of which Measures is a good example.
 
Hernan: "But I think that if you have a Duration and want its value in
seconds you will have to send it the message #asSeconds or something like
that... Maybe I'm wrong, but if that is the case I prefer to have more
polymorphic objects (measures) and less messages, for me is easier to
remember."
 
Yes, you have to send messages such as #asSeconds or #asHours to a
durational object (as required by the ANSI Smalltalk Standard.)
 
And as I said, Measures are a great idea--especially if they provide
Frink-like functionality (http://futureboy.homeip.net/frinkdocs/)
 
Hernan: "I tried "10 * CalendarDayDuration" and it did not work... so they
do not conform to the commutative property that one expect from arithmetic
operations... We face those problems when doing Aconcagua, that's why we
create a new abstraction called ArithmeticObject (that if we could, Number
should subclass it). "
 
Implementing the commutative property, while highly desirable, has one
drawback: it exposes Chronos to the danger of contention with the methods
added to base-library classes by others to do the same for their date/time
objects. If Chronos were just an application whose code was not meant to be
widely shared among many different applications and other libraries, I'd
have implemented the commutative property for all appropriate cases.  The
case for library code is much more problematical--and this problem is a high
priority issue to be solved by the language designers.
 
Hernan: "But this examples you provide do not have the same result as in
Chalten."
 
At first, I didn't understand why you said that.  Then I realized it was
because the Chalten expresssions were evaluating to DayOfTheWeek objects,
and not to Date objects. My mistake.
 
Chronos doesn't have "DayOfTheWeek" objects.  It does define the constants
Sunday, Monday, etc. containing the integers 1, 2, etc.  It also uses the
Symbols #Sunday, #Monday, etc. One reason is because the ANSI Smalltalk
Standard defines dayOfWeek as returning an integer between 1 and 7, whith 1
= Sunday.
 
Hernan: "This example also is not the same as what I showed with Chalten. My
example return a Month, in this case "February" that is instance of
GregorianMonth..."
 
Again, Chronos doesn't have a "Month" object that exactly corresponds to
Chalten's.  It does define the constants January, February, etc containing
the integers 1, 2 etc.  It also uses the Symbols #January, #February etc.
The ANSI Standard is one reason. Another is to preserve polymorphism across
different calendars.
 
Chronos also has the class MonthOfYear--but it's an AnnualDate, which
although similar in some ways to the Chalten concept, is also different
Then there's MonthlyCalendar, which again is different, since it's a
singleton that encapsulates all knowledge regarding the characteristics of a
particular month in a partcicular type of year for a particular calendar
(e.g. the Hebrew month 'Adar Rishon' in a Hebrew deficient leap year--there
are six different year types in the Hebrew calendar; 'Adar Rishon' is a leap
month; Hebrew years may have either 12 or 13 months.)
 
For most cases, my belief is that a Timeperiod with a duration of 1 month
provides all needed functionality to represent a month in the timeline, and
a Timeperiod with duration 1 year does the same for the representation of a
year in the timeline.  But I'm open to counterexamples.  And I have no
objection to having objects other than Timeperiods that can represent "a
month in the timeline" or "a year in the timeline" available as optional
extensions to Chronos.
 
Hernan: "we just believe that doing so you loose polymorphisms and we did
not what to do that. I mean, you can not use the + with number and duration
polymorphically. Instead in Aconcagua, the +, - , etc. can be used with
number and measures polymorphically."
 
Well, I haven't tried it, but my hunch would be that double-dispatching
should enable the desired polymorphism.  I'll look into the matter at some
point.
 
--Alan
 
P.S.  I'm sure we'll have more to talk about "as time goes by." (Heh)


  _____  

From: squeak-dev-bounces at lists.squeakfoundation.org
[mailto:squeak-dev-bounces at lists.squeakfoundation.org] On Behalf Of Hernan
Wilkinson
Sent: Friday, April 21, 2006 5:49 AM
To: The general-purpose Squeak developers list
Subject: Re: The Timing of Time


Alan Lovejoy wrote: 

Jeff, Hernan:



I also will put my comments in-line, following Hernan's, preceded by ##:



-----Original Message-----

From: squeak-dev-bounces at lists.squeakfoundation.org

[mailto:squeak-dev-bounces at lists.squeakfoundation.org] On Behalf Of Hernan

Wilkinson

Sent: Thursday, April 20, 2006 3:20 PM

To: The general-purpose Squeak developers list

Subject: Re: The Timing of Time



Hi Jeff, I will answer your questions over Alan's responses to make it easy

to compare both solutions. (I hope Alan you don't mind...)



Alan Lovejoy wrote:

[snip]

  

Date equality works correctly between different calendars, and

Timepoint equality works correctly between different time zones and/or

different calendars.





    

Chalten does not have support for time zone or different calendars. It just

support the Gregorian Calendar; we are planning to add support to other

calendars  and time zone but in different packages... it is not common to

use other calendars than the Gregorian and we don't want to make the most

commonly used abstraction difficult to understand because of that support.



## Non-gregorian calendars in Chronos are completely invisible to

programmers who don't want or need them. In fact, all the non-Gregorian

calendar classes could simply be removed, and the only Chronos code that

would break would be example code in comments (if you can call such

code-in-the-comments "Chronos code.")  Where in any of the Chronos examples

you've seen in this discussion can you spot the fact that Chronos even

supports calendars other than the Gregorian?  I mention it in passing, but

there's no evidence of it in any of the code examples.

  

You are right, but I did not say that Chronos had that problem, I just said
what we wanted to avoid, that's all... 


## Why not just use Chronos for time zones and for non-Gregorian calendars?

I've already done the hard work--and you'll find the job is a lot harder

than you currently imagine. The opportunity for synergy between Chalten and

Chronos in this area seems very strong to me.

  

I agree. As I said in other mails:
1) We have to add that functionality to Chalten because it is part of the
thesis that Maxi has to write. It would be of great help to "use" what you
did with Chronos, to merge your ideas with ours, but first we have to
analyze Chronos, understand how it is design and implemented, etc. So, yeah,
I agree with what you say. I'll let you know personally (I think it does not
make sense to use the squeak list for this) what are the next steps once we
decide them 
2) I believe, as you say, that we and all the smalltalk community can
benefit from merging Chronos and Chalten, so if I say something that Chalten
does is not to say that Chronos does not, I don't know how Chronos is
implemented, all the functionality it covers, etc.


  

It's easy to convert a Chronos YearMonthDay or Timepoint into a count

of days, or to convert a count of days into a YearMonthDay or Timepoint:



	YearMonthDay daysSinceEpoch: YearMonthDay today daysSinceEpoch.







    

Same for Chalten. For example:

   ( January first, 2006) numberOfDaysFromBaseDate --> Returns  "38351

days".



But as you can see it returns a Measure. They also are immutable,

comparable, can be put in Sets, Dictionaries, etc.

I believe that the fact that we return a Measure and not just a numbers

makes "explicit" the meaning of the returned object. It is not just

38351 but "38351 days"



##  Chronos:

##

##		1) YearMonthDay today - (YearMonthDay year: 1901 day: 1) =>

38460:00:00:00 (38460 days as a duration object)

##		2) YearMonthDay today civilDurationSince: (YearMonthDay

year: 1901 day: 1)=> P105Y3M19D (also a duration object, representing 105

years, 3 months, 19 days)



## Chronos durational objects are immutable, comparable, can be used in

Sets, Dictionaries, etc.

  

Defenitly you know more than we know about the time domain. We have not gone
into too much detail on the civil calendars, etc. But I like the idea of
using measure instead of duration because as conceptual entities, they
represent the same. The only difference is that measures are more generic
because they can be use as measure of time, length, money, etc. So, I like
the idea of using measure because when you get used to it, you don't have to
care anymore about all the different classes that represent, in an abstract
way, the same concept, like Duration, Money, Length, etc. All those concept
can be represented as Measures. But hey, that's my idea and not everybody
has to agree with me...


What's the advantage? Well, there are many, for example:

    1) If you see the message #numberOfDaysFromBaseDate we, as programmer,

know that it will return a number or a measure in this case, but what

happens if you just see the object "38351"? You will not be able to know if

they are days, seconds, years or what ever meaning it has. If instead you

see "38351 days", now you know you are dealing with days. But not only we as

programmers will notice that, the whole idea of Aconcagua is that the

computer will handle any arithmetic mistake you can have mixing measure of

different units. Using measures, you will never mix days and years. For

example:

          a) If the model returns numbers instead of measures, you could

write:

            ( January first, 2006) numberOfDaysFromBaseDate + ( January

first, 2006) year yearsFromBaseDate --> That is 38351 + 106 =  40357

            There is no way to know from the result that you added days and

years.

          b) If the model returns measures:

            ( January first, 2006) numberOfDaysFromBaseDate + ( January

first, 2006) year yearsFromBaseDate --> It will return "38351 days + 106

years"

           As you can see, it does not mix up days and years because they

are not interchangeable. A year can be 365 or 366 days in the Gregorian

calendar.

    2) Because we use measures, if you want to know the number of hours from

the base date, you can do:

       ( January first, 2006) numberOfDaysFromBaseDate convertTo:

TimeUnits hour --> Returns  " 920424 hours".

       Or seconds:

       ( January first, 2006) numberOfDaysFromBaseDate convertTo:

TimeUnits hour --> Returns  " 3313526400 seconds"

       There is no need for a class Duration or special messages to get the

number of seconds from base date, etc.



## I generally agree with Hernan's explanation of the motivation for using

durational objects.  But there are times when integer numbers are better.

For one thing, the messages to get the count of days must exist in order to

create the durational values. And whith a Chrohos durational object, there

is no need to convert it from days to hours--the same object encompasses all

units from nanoseconds to weeks.

  

But I think that if you have a Duration and want its value in seconds you
will have to send it the message #asSeconds or something like that... Maybe
I'm wrong, but if that is the case I prefer to have more polymorphic objects
(measures) and less messages, for me is easier to remember.


## By the way, if you are intrigued by the Aconcagua domain-specific

language for measures, you should also check out Frink:

http://futureboy.homeip.net/frinkdocs/

  

Ok, thanks!. I'll check that out


  

So the problem of indexing by a date resolves to the problem of

indexing by an integer. It's also possible to convert a date into a

count of months or years since the calendar epoch, and to convert a

count of months or years since a calendar epoch into a date.



Both YearMonthDay and Timepoint (which inherits from YearMonthDay)

support date and time arithmetic operations:



	#addingYears:

	#subtractingYears:

	#addingMonths:

	#subtractingMonths:

	#addingDays:

	#subtractingDays:

	#addingYears:months:days:

	#subtractingYears:months:days:

	#addingHours:

	#subtractingHours:

	#addingMinutes:

	#subtractingMinutes:

	#addingSeconds:

	#subtractingSeconds:

	#addingSeconds:nanonseconds:

	#subtractingSeconds:nanoseconds:







    

Well, here is an example of how we simplified the protocol of all

PointInTime objects using measures. For the same porpouse that Alan shows in

Chronos, we have only two messages: #next: and #previous: (we don't use add

and subtract for semantinc issues, but that another problem it does not make

sense to talk about now). For example:



    GregorianDate today next: 10 * TimeUnits day   -> Will return "

April 30, 2006" if today is " April 20, 2006"

## YearMonthDay today + (CalendarDayDuration * 10)

  

Cool, I'm learning about Chronos. From the functionality point of view, they
are equal, but for me is difficult to understand at first sight what
CalendarDayDuration mean. I understand now, but I tried "10 *
CalendarDayDuration" and it did not work... so they do not conform to the
commutative property that one expect from arithmetic operations... We face
those problems when doing Aconcagua, that's why we create a new abstraction
called ArithmeticObject (that if we could, Number should subclass it). 
Anyway, as I said before, I believe that Measure is a more generic and
abstract way of representing time durations and the good thing about
measures is that all measures are polymorphic!. The same is not true for
Duration, Money (if it existed), Length (if it is created) etc. 

[I erased some of the examples because they are equivalent to the one above]


    GregorianDay today next: 3 * TimeUnits day --> Will return "Sunday"

if today is  "Thursday"

## YearMonthDay today + (CalendarDayDuration * 3)

## Timepoint today + (CalendarDayDuration * 3)

## Timeperiod today + (CalendarDayDuration * 3)

  

But this examples you provide do not have the same result as in Chalten.

YearMonthDay today + (CalendarDayDuration * 3) --> Returns an instance of
YearMonthDay instead of an object that represent a day in a week, like in
Chalten that returns "Thursday", and instance of GregorianDay (it is not the
same as GregorianDate).

The other two examples do not return a day also. (By the way, what is the
difference between YearMonthDay and Timepoint? I see you use it in a
different way as we do...)


    GregorianMonth current previous: 2 * TimeUnits month --> Will return "

February" if the current month is " April"

    etc.

## Timeperiod currentMonth - (MonthDuration * 2)

  

This example also is not the same as what I showed with Chalten. My example
return a Month, in this case "February" that is instance of
GregorianMonth...

[snip]


We do not implemented the #+, #-, etc. messages in PointInTime objects

because those message do not keep the same semantic as the arithmetic

message. For example. if you do "1 + 2" you get "3". That is, you are adding

number, you get a number. But for dates is not the same... For example

"GregorianDate today + 10 days" would not return a measure of days but a new

date, so it does not match the semantics of the arithmetic +.

Of course this is a decision we made and not everybody could like or agree

with that... but we believe it makes more clear the model's language.



## Chronos implements the #+ and #- messages for two reasons:

##

## 1) It's required by the ANSI Smalltalk Standard.

## 2) In mathematics, the semantics of + and - depend upon the algebra one

formally defines. There is no general requirement that both operands of the

+ or - opertor be of the same type. "Mixed mode" arithmetic expressions are

common, useful and formally well defined.

  

That's find. When we did Chalten we decided not to follow the ANSI standard
because we do not agree with some abstractions they propose.
About the mathematic semantics, as you say, you can define what ever you
want for the + and -, we just believe that doing so you loose polymorphisms
and we did not what to do that. I mean, you can not use the + with number
and duration polymorphically. Instead in Aconcagua, the +, - , etc. can be
used with number and measures polymorphically.


  

The "duration" instance variable might have the value

"ScientificDuration

hours: 3.5", or it might have the value "CalendarDuration months: 3."

The "baseTimepoint" instance variable could hold either a Timepoint, a

YearMonthDay--or even a Timeperiod (interval of time.) For example,

"Timeperiod currentMonth + (CalendarDuration days: 5)" (if executed

during the month of April, 2006) evaluates to "2006-04-06/P1M" (the

one-month interval starting 6 April 2006.)





    

Chalten also have an abstraction for this, it named Timespan. It represents

segment of the time line. For example:



    GregorianTimespan from: GregorianDate today duration: 20 * TimeUnits day

--> Returns  "20 days from April 20, 2006"

## Timeperiod from: YearMonthDay today duration: CalendarDayDuration * 20 =>

2006-04-20/P20D (ISO 8601 notation for "the 20-day period starting with

2006-04-20")

  

Definitively you have more idea about time standards that we do....


    GregorianTimespan from: GregorianYear current duration: 10 * TimeUnits

year --> Returns  " 10 years from Year 2006"

## Chronos doesn't have a "Year" class -- and that's one of the obvious

opportunities for synergy between Chalten and Chronos

We also have the abstraction GregorianDay (ie. Monday) , GregorianDayOfMonth
(ie. January 1) and GregorianMonthOfYear (January 2006)
Just to understand a little bit more about Chronos, does it have those
abstractions?

## It's also serendipitous that both Timeperiod and GregorianTimespan use


#from:duration: as instance creation methods--must be the right method name.

  

jaja. I agree....



    GregorianTimespan from: GregorianMonth current duration: 2 * TimeUnits

month --> Returns  " 2 months from April"

## Yet another opportunity for synergy...

  

Ok, I made a question before about this... It is answered now.


    GregorianTimespan from: GregorianDay today duration: 2 * TimeUnits day

--> Returns  " 2 days from Thursday"

    GregorianTimespan from: GregorianDayOfMonth today duration: 25 *

TimeUnits day --> Returns  " 25 days from April 20"

## Timeperiod from: DayOfMonth today duration: CalendarDayDuration * 25

## Timeperiod from: DayOfYear today duration: CalendarDayDuration * 25

## Timeperiod from: WeekOfMonthDayOfWeek today duration: CalendarDayDuration

* 25

## Timeperiod from: GregorianEaster canonical duration: CalendarDayDuration

* 25



    GregorianTimespan from: GregorianMonthOfYear current duration: 12 *

TimeUnits month --> Returns  " 12 months from April of Year 2006"

## Yet another opportunity for synergy...

  

Ok, cool.


    GregorianTimespan from: TimeOfDay now duration: 12 * TimeUnits hour

--> Returns  " 12 hours from 18:53:33" (I wrote the mail at 18:53:33)

## Uh, oh: Both Chronos and Chalten use the class name TimeOfDay. This won't

matter in VW, but Squeak really needs namespaces or class boxes...

## Timeperiod from: TimeOfDay now duration: 12 * HourDuration

  

It is good to see we use the same names...


    GregorianTimespan from: GregorianDateTime now duration: 12 * TimeUnits

hour --> Returns  " 12 hours from April 20, 2006 18:54:14"



Look how easy is to use the same abstraction, GregorianTimespan, to any type

of PointInTime. I think we could achieve that because we are using measures

and all point in times are polymorphic. (By the way, we use

GregorianTimespan because Chronology already defines Timespan) We also have

time intervals, that are instances of the class "ArihtmeticObjectInterval"

(I think not a good name). We could not use the Interval class for many

reason. Anyway, ArihtmeticObjectInterval is an interval of any type of

Magnitude, for example Measures, Numbers and of course PointInTimes. For

example:

    GregorianDate today to: December first, 2006 --> Returns an interval

from today to December fist 2006 by 1 day

    GregorianDate today to: December first, 2006 by: 10 * TimeUnits day

--> Returns an interval from today to December fist 2006 with steps of

10 days



## I considered implementing a TemporalInterval where the step value would

be specified.  In the end, I decided to simply make it easy to enumerate

between the endpoints of a Timeperiod using any ScientificDuration or

CivilDuration as the step value, and let applications manage the step value

as they care to.  Implementing a TemporalInterval on top of Timeperiod would

be easy to do.  Perhaps this is another opportunity for synergy between

Chronos and Chalten.



    GregorianMonth current to: December by: 2 * TimeUnits month --> Now,

and interval of months.... the same for other time points...

## Yet another opportunity for synergy...

  

Cool.... we have a lot of thing to talk about... Maxi is taken notes on this
so we can take advantage of what we have talk so far


Because these objects are intervals, they are polymorphic with Collection:



    (GregorianDate today to: December first, 2006) select: [ :aDate |

aDate isMonday ] --> Returns all Mondays from today to December first

    (GregorianYear current to: (GregorianYear number: 3000) ) select: [

:aYear | aYear isLeap ] --> Returns all leap year up to year 3000

    (January first, 2006 to: GregorianDate today) collect: [ :aDate |

aDate distanceTo: GregorianDate today ] --> Returns an array with

measures from 109 days to 0 days



  

Does that answer your question about indexing?



Jeff: "What should happen if you start at January 28'th and advance by a

month and then go back by a month?

Jan28 --> Feb28 --> (Jan 28 or Jan 31?)   Can we agree on some

conventions? "



(YearMonthDay year: 2006 month: 1 day: 28) + 1 months =>  2006-02-28

(YearMonthDay year: 2006 month: 1 day: 31) + 1 months =>  2006-02-28

(YearMonthDay year: 2006 month: 1 day: 28) + 1 months - 1 months  =>

2006-01-28

(YearMonthDay year: 2006 month: 1 day: 31) + 1 months - 1 months  =>

2006-01-28



(Note: "1 months" works in VW and Dolphin, but not in Squeak, where

"(CalendarDuration months: 1)" must be used instead--or you could add a

#months and #years method to Integer yourself.)





    

In Chalten we have a set of units that are interchangeable for years,

moths, decades, etc. and another for days, hours, seconds, weeks, etc.

Notice that time units are not the same as time points (there is an

object for years, ie. GregorianYear, and there is a unit to measure

years, ie. TimeUnit year)

That means that measures expressed in days can not be converted to

months or years and vicersa. (Is 30 days a month? or 31 days? etc.)

Because of the irregularity of the Gregorian calendar we decided not to

allow movements of point in times with measure of units of less

granularity that the point in time granularity. For example:

    GregorianDate today next: 1 * TimeUnits second --> Is not allowed

But we added for convenience that possibility of doing:

    (January thirtyfirst, 2006 next: 1 * TimeUnits month) --> Returns

February 28, 2006

    (January thirtyfirst, 2006 next: 1 * TimeUnits month) previous: 1 *

TimeUnits month --> Returns January 28, 2006

I don't like this behavior but we added because we people are used to it.





## YearMonthDay today addingSeconds: 1000 => 2006-04-20T00:16:40



## This case is no different than "3 / 4" or "100 factorial."

Autoconvervion/autopromotion is the Smalltalk way.

  

Ok, your solution is different.
Well, Alan, this helped us to understand more about Chronos... as you say we
should find the way to merge the models. I'll let you know what our ideas
are as soon as we decide them. Let's keep in touch.

Bye,
Hernan.


Well, the mail became longer that I expected. I hope it helps you to

understand Chalten and also compare it with other solutions.



Bye,

Hernan



  

Chronos implements date arithmetic to satisfy the typical business use

    

case.

  

If you want "scientific" behavior, use a ScientificDuration.  If you want a

"month" to always be 30 days, use a "monthDuration" value defined as

"CalendarDuration days: 30."



But these questions just scratch the surface.  There's a lot more than can

and should be asked.



--Alan



-----Original Message-----

From: squeak-dev-bounces at lists.squeakfoundation.org

[mailto:squeak-dev-bounces at lists.squeakfoundation.org] On Behalf Of Jeffrey

J. Hallman

Sent: Wednesday, April 19, 2006 9:10 AM

To: squeak-dev at lists.squeakfoundation.org

Subject: Re: The Timing of Time



I'm enjoying the discussion about calendars and times, but I'd like to

direct some attention to the idea of a TimeIndex.  I came at this from

another direction, thinking of a TimeSeries as data indexed by time, which

leads to the idea of a TimeIndex.



In my current implementation, a TimeIndex has two instance variables: a

    

freq

  

symbol (such as #weeklyMonday, #monthly, #hourly, etc.) and an integer

period, which represents the number of periods elapsed since the base

    

period

  

for that freq. TimeIndex understands '+' and '-', so if 'z'

is the weeklyMonday time index for April 17 2006, then (z - 4) asYmd yields

20060320, and so on.



My TimeSeries class is a subclass of Matrix with an additional instance

variable called 'start' which is a TimeIndex, and has accessors like



TimeSeries>>atTimeIndex:

TimeSeries>>atTimeIndex:put:

TimeSeries>>atTimeIndex:column:

TimeSeries>>atTimeIndex:column:put:



I've found this scheme works pretty well, and will probably generalize it a

bit more by making TimeIndex an abstract class with subclasses for

    

different

  

kinds of sequences.



I think a package that wants to handle date and time issues should have

something like a TimeIndex in it that allows sequences of various

frequencies to be defined.  At the same time, the implementation should be

simple enough to be easily understood, and indexing into a TimeSeries

    

should

  

be very fast.  If indexing isn't fast, we'll end up with ugly code in every

TimeSeries method that that converts TimeIndex'es to row numbers and back.

Trust me, you really don't want that.



So how do these various packages handle indexing data by time?



A second issue is how to handle date arithmetic.  Some durations are based

on linear time (e.g., weeks, seconds, hours, etc.) and others are based on

the calendar (months, years, etc.).  The distinction between these two

    

leads

  

to questions like: What should happen if you start at January 28'th and

advance by a month and then go back by a month?

Jan28 --> Feb28 --> (Jan 28 or Jan 31?)   Can we agree on some

conventions?



Jeff Hallman















    





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______________________________

Lic. Hernán A. Wilkinson

Gerente de Desarrollo y Tecnología

Mercap S.R.L.

Tacuari 202 - 7mo Piso - Tel: 54-11-4878-1118

Buenos Aires - Argentina

http://www.mercapsoftware.com

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