Ubuntu Manpage: Date::Manip::Misc - Miscellaneous information about Date::Manip

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       Date::Manip::Misc - Miscellaneous information about Date::Manip

SHOULD I USE DATE::MANIP

If you look in CPAN, you'll find that there are a number of Date and Time packages. Is
Date::Manip the one you should be using? That isn't a trivial question to answer. It
depends to a large extent on what you are trying to do.

Date::Manip is certainly one of the most powerful of the Date modules (the other main
contender being the DateTime suite of modules). I'm trying to build a library which can
do _EVERY_ conceivable date/time manipulation that you'll run into in everyday life
dealing with the Gregorian calendar. To the best of my knowledge, it will do everything
that any other date module will do which work with the Gregorian calendar, and there are a
number of features that Date::Manip has that other modules do not have.

There is a tradeoff in being able to do "everything"... and that tradeoff is primarily in
terms of performance. Date::Manip is written entirely in Perl and is the largest of the
date modules. Other modules tend to be faster than Date::Manip, and modules written in C
are significantly faster than their Perl counterparts (at least if they're done right).
Although I am working on making Date::Manip faster, it will never be as fast as other
modules. And before anyone asks, Date::Manip will never be translated to C (at least by
me). I write C because I have to. I write Perl because I like to. Date::Manip is
something I do because it interests me, not something I'm paid for.

If you are going to be using the module in cases where performance is an important factor,
and you're doing a fairly small set of simple date operations over and over again, you
should carefully examine the other date modules to see if they will meet your needs.

Date::Manip does NOT provide functionality for working with alternate calendars such as
the Chinese or Hebrew calendars, so if you need that functionality, you definitely need to
look elsewhere (the DateTime suite probably).

On the other hand, if you want one solution for all your date needs, don't need peak
speed, or are trying to do more exotic date operations, Date::Manip is for you.
Operations on things like business dates, foreign language dates, holidays and other
recurring events, complete timezone handling, etc. are available more-or-less exclusively
in Date::Manip. At the very least, if you want to be able to do these operations, it will
require using several other modules, each with it's own interface. Also, when you work
with Date::Manip, you work with one author and one module. The DateTime suite currently
consists of almost 100 modules and 75 authors.

In addition, I am making significant performance improvements in Date::Manip. Although it
will never be as fast as some of the other perl modules, I believe that it is already
competitive enough for most purposes, and I continue to look for places where I can
improve performance, so performance should improve over time.

YEAR 2000 AND YEAR 2007 DST CHANGE

       Did Date::Manip have any problems with Y2K compliance? Did it have any problems with the
       revised daylight saving time changes made in 2007?

       Although Date::Manip will parse many date strings (including dates with 2-digit years),
       internally they are stored as a 4 digit year, and all operations are performed using this
       internal representation, so Date::Manip had no problems with the Y2K issue. Of course,
       applications written which stored the year as 2 digits (whether or not it used
       Date::Manip) may have had problems, but they were not because of this module.

       Similarly for the 2007 changes in daylight saving time made in the United States,
       Date::Manip was not affected. Date::Manip makes use of the current time zone, but it gets
       that information from the operating system the application is running on. If the operating
       system knows about the new daylight saving time rules... so does Date::Manip.

WHAT DATES ARE DATE::MANIP USEFUL FOR?

Date::Manip applies to the Gregorian calendar. It does not support alternative calendars
(Hebrew, Mayan, etc.) so if you want to use an alternative calendar, you'll need to look
elsewhere.

The Gregorian calendar is a relatively recent innovation. Prior to it, the Julian calendar
was in use. The Julian calendar defined leap years as every 4th year. This led to
significant calendar drift over time (since a year is NOT 365.25 days long). It was
replaced by the Gregorian calendar which improved the definition of leap years, and at
that point, the calendar was adjusted appropriately.

Date::Manip extrapolates the Gregorian calendar back to the year 0001 AD and forward to
the year 9999 AD, but that does not necessarily mean that the results are useful. As the
world adopted the Gregorian calendar, the dates using the Julian calendar had to be
changed to fit to account for the drift that had occurred. As such, the dates produced by
Date::Manip in an era where the Julian calendar was in use do not accurately reflect the
dates actually in use. In historical context, the Julian calendar was in use until 1582
when the Gregorian calendar was adopted by the Catholic church. Protestant countries did
not accept it until later; Germany and Netherlands in 1698, British Empire in 1752, Russia
in 1918, etc. Date::Manip is therefore not equipped to truly deal with historical dates
prior to about 1600, and between 1600 and 1900, the calendar varied from country to
country.

A second problem is that the Gregorian calendar is itself imperfect and at some point may
need to be corrected (though it's not clear that this will happen... drift may now be
accounted for using leap seconds which means that the Gregorian calendar may be useful
indefinitely). No attempt is made to correct for the problems in the Gregorian calendar
for a couple reasons. First is that my great great great grandchildren will be long dead
before this begins to be a problem, so it's not an immediate concern. Secondly, and even
more importantly, I don't know what the correction will be (if any) or when it will be
implemented, so I can safely ignore it.

There is some limitation on how dates can be expressed such that Date::Manip can handle
them correctly. Date::Manip stores the year internally as a 4-digit number. This is
obviously not a limit due to the Gregorian calendar, but I needed a way to store the dates
internally, and the 4-digit year was chosen. I realize that the 4-digit limitation does
create a time when it will break (quite similar to those who chose a 2-digit
representation set themselves up for the Y2K problem). Frankly, I'm not too concerned
about this since that date is 8000 years in the future! Date::Manip won't exist then.
Perl won't exist then. And it's quite possible that the Gregorian calendar won't exist
then. That's a much different situation than the Y2K choice in which programmers chose a
representation that would break within the lifetime of the programs they were writing.

Given the 4-digit limitation, Date::Manip definitely can't handle BC dates, or dates past
Dec 31, 9999. So Date::Manip works (in theory) during the period Jan 1, 0001 to Dec 31,
9999. There are a few caveats:

Gregorian calendar issue
In practical terms, Date::Manip deals with the Gregorian calendar, and is most useful
in the period that that calendar has been, or will be, in effect. As explained above,
the Gregorian calendar came into universal acceptance in the early 1900's, and it
should remain in use for the foreseeable future.

So... in practical terms, Date::Manip is probably useful from around 1900 through
several thousand years from now.

First/last week
In one part of the code (calculating week-of-year values), Date::Manip references
dates one week after and one week before the date actually being worked on. As such,
dates during the first week in the year 0001 fail (because a week before is in the
year 1 BC), and those in the last week in the year 9999 fail (because a week later is
in 10,000).

No effort will be made to correct this because the added functionality is simply not
that important (to me), especially since the Gregorian calendar doesn't really apply
in either instance. To be absolutely safe, I will state that Date::Manip works as
described in this manual during the period Feb 1, 0001 to Nov 30, 9999, and I will
only support dates within that range (i.e. if you submit a bug using a date that is
not in that range, I will will consider myself free to ignore it).

Leap seconds
Date::Manip does NOT make use of the leap seconds in calculating time intervals, so
the difference between two times may not be strictly accurate due to the addition of a
leap second.

Three-digit years
Date::Manip will parse both 2- and 4-digit years, but it will NOT handle 3 digit
years. So, if you store the year as an offset from 1900 (which is 3 digits long as of
the year 2000), these will NOT be parseable by Date::Manip. Since the perl functions
localtime and gmtime DO return the year as an offset from 1900, the output from these
will need to be corrected (probably by adding 1900 to the result) before they can be
passed to any Date::Manip routine.

FUTURE IDEAS

       A number of changes are being considered for future inclusion in Date::Manip.  As a rule,
       the changes listed below are not finalized, and are open to discussion.

       Rewrite parsing for better language support
           Currently, all of Date::Manip's parsing is based on English language forms of dates,
           even if the words have been replaced by the equivalent in some other language.

           I am considering rewriting the parsing routines in order to allow date forms that
           might be used in other languages but do not have a common English equivalent, and to
           account for the fact that some English formats may not have an equivalent in another
           language.

       Adding granularity
           The granularity of a time basically refers to how accurate you wish to treat a date.
           For example, if you want to compare two dates to see if they are identical at a
           granularity of days, then they only have to occur on the same day.  At a granularity
           of an hour, they have to occur within an hour of each other, etc.

           I'm not sure how useful this would be, but it's one of the oldest unimplemented ideas,
           so I'm not discarding it completely.

ACKNOWLEDGMENTS

There are many people who have contributed to Date::Manip over the years that I'd like to
thank. The most important contributions have come in the form of suggestions and bug
reports by users. I have tried to include the name of every person who first suggested
each improvement or first reported each bug. These are included in the
Date::Manip::Changes5 and Date::Manip::Changes6 documents. The list is simply too long to
appear here, but I appreciate their help.

A number of people have made suggestions or reported bugs which are not mentioned in these
documents. These include suggestions which have not been implemented and people who have
made a suggestion or bug report which has already been suggested/reported by someone else.
For those who's suggestions have not yet been implemented, they will be added to the
appropriate Changes document when (if) their suggestions are implemented. I keep every
single suggestion I've ever received and periodically review the unimplemented ones to see
if it's something I'm interested in, so even suggestions made years in the past may still
appear in future versions of Date::Manip, and the original requester will be attributed at
that point (some of the changes made to Date::Manip 6.00 were based on suggestions 10
years old which never fit in with version 5.xx, but which I knew I wanted to implement).
For those who have sent in requests/reports that had been previously made by someone else,
thank you too. I'd much rather have a suggestion made twice than not at all.

Thanks to Alan Cezar and Greg Schiedler for paying me to implement the Events_List
routine. They gave me the idea, and were then willing to pay me for my time to get it
implemented quickly.

I'd also like to thank a couple of authors. Date::Manip has gotten some really good press
in a couple of books. Since no one's paying me to write Date::Manip, seeing my module get
a good review in a book written by someone else really makes my day. My thanks to Nate
Padwardhan and Clay Irving (Programming with Perl Modules -- part of the O'Reilly Perl
Resource Kit); and Tom Christiansen and Nathan Torkington (The Perl Cookbook). Also,
thanks to any other authors who've written about Date::Manip who's books I haven't seen.

I'd also like to thank the people who are maintaining the zoneinfo database (and who
replied quickly to several inquiries).

I have borrowed from other modules. I originally borrowed the code for determining if a
year was a leap year from code written by David Muir Sharnoff. I borrowed many of the
original date printf formats from code written by Terry McGonigal as well as the Solaris
date command. More recently, I borrowed the code to do time zone registry lookups on
Windows from the DateTime-TimeZone module, though I rewrote it to work better with
Date::Manip.

BUGS AND QUESTIONS

       Please refer to the Date::Manip::Problems documentation for information on submitting bug
       reports or questions to the author.

LICENSE

       This script is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.

AUTHOR

       Sullivan Beck (sbeck@cpan.org)