Date::Manip::Misc(3) User Contributed Perl Documentation Date::Manip::Misc(3)NAMEDate::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
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
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 365.24 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.
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, the first week in the year
0001 fail (because a week before is in the year 1 BC), and 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).
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.
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.
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.
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.
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.
Date::Manip - main module documentation
This script is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
Sullivan Beck (firstname.lastname@example.org)
perl v5.16.3 2014-04-30 Date::Manip::Misc(3)