phpman > perldoc > Date::Manip::Misc(3pm)

NAME
    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.

SEE ALSO
    Date::Manip - main module documentation

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 AT cpan.org)