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[wxWidgets.git] / docs / doxygen / overviews / datetime.h

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// Name:        datetime.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
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/*!

 @page overview_datetime Date and time classes overview

 Classes: #wxDateTime, #wxDateSpan, #wxTimeSpan, #wxCalendarCtrl

 @li @ref overview_datetime_introduction
 @li @ref overview_datetime_classes
 @li @ref overview_datetime_characteristics
 @li @ref overview_datetime_timespandiff
 @li @ref overview_datetime_arithmetics
 @li @ref overview_datetime_timezones
 @li @ref overview_datetime_dst
 @li @ref overview_datetime_holidays
 @li @ref overview_datetime_compat


 <hr>


 @section overview_datetime_introduction Introduction

 wxWidgets provides a set of powerful classes to work with dates and times. Some
 of the supported features of #wxDateTime class are:

 @li Wide range: the range of supported dates goes from about 4714 B.C. to
     some 480 million years in the future.

 @li Precision: not using floating point calculations anywhere ensures that
     the date calculations don't suffer from rounding errors.

 @li Many features: not only all usual calculations with dates are supported,
     but also more exotic week and year day calculations, work day testing, standard
     astronomical functions, conversion to and from strings in either strict or free
    format.

 @li Efficiency: objects of wxDateTime are small (8 bytes) and working with
     them is fast



 @section overview_datetime_classes All date/time classes at a glance

 There are 3 main classes declared in @c wx/datetime.h: except #wxDateTime itself 
 which represents an absolute moment in time, there are also two classes -
 #wxTimeSpan and #wxDateSpan - which represent the intervals of time.

 There are also helper classes which are used together with wxDateTime:
 #wxDateTimeHolidayAuthority which is used to determine whether a given date 
 is a holiday or not and #wxDateTimeWorkDays which is a derivation of this
 class for which (only) Saturdays and Sundays are the holidays. See more about
 these classes in the discussion of the #holidays.

 Finally, in other parts of this manual you may find mentions of wxDate and
 wxTime classes. @ref overview_datetime_compat are obsolete and
 superseded by wxDateTime.



 @section overview_datetime_characteristics wxDateTime characteristics

 #wxDateTime stores the time as a signed number of
 milliseconds since the Epoch which is fixed, by convention, to Jan 1, 1970 -
 however this is not visible to the class users (in particular, dates prior to
 the Epoch are handled just as well (or as bad) as the dates after it). But it
 does mean that the best resolution which can be achieved with this class is 1
 millisecond.

 The size of wxDateTime object is 8 bytes because it is represented as a 64 bit
 integer. The resulting range of supported dates is thus approximatively 580
 million years, but due to the current limitations in the Gregorian calendar
 support, only dates from Nov 24, 4714BC are supported (this is subject to
 change if there is sufficient interest in doing it).

 Finally, the internal representation is time zone independent (always in GMT)
 and the time zones only come into play when a date is broken into
 year/month/day components. See more about #timezones below.

 Currently, the only supported calendar is Gregorian one (which is used even
 for the dates prior to the historic introduction of this calendar which was
 first done on Oct 15, 1582 but is, generally speaking, country, and even
 region, dependent). Future versions will probably have Julian calendar support
 as well and support for other calendars (Maya, Hebrew, Chinese...) is not
 ruled out.



 @section overview_datetime_timespandiff Difference between wxDateSpan and wxTimeSpan

 While there is only one logical way to represent an absolute moment in the
 time (and hence only one wxDateTime class), there are at least two methods to
 describe a time interval.

 First, there is the direct and self-explaining way implemented by
 #wxTimeSpan: it is just a difference in milliseconds
 between two moments in time. Adding or subtracting such an interval to
 wxDateTime is always well-defined and is a fast operation.

 But in the daily life other, calendar-dependent time interval specifications are
 used. For example, 'one month later' is commonly used. However, it is clear
 that this is not the same as wxTimeSpan of 60*60*24*31 seconds because 'one
 month later' Feb 15 is Mar 15 and not Mar 17 or Mar 16 (depending on whether
 the year is leap or not).

 This is why there is another class for representing such intervals called
 #wxDateSpan. It handles these sort of operations in the
 most natural way possible, but note that manipulating with intervals of
 this kind is not always well-defined. Consider, for example, Jan 31 + '1
 month': this will give Feb 28 (or 29), i.e. the last day of February and not
 the non-existent Feb 31. Of course, this is what is usually wanted, but you
 still might be surprised to notice that now subtracting back the same
 interval from Feb 28 will result in Jan 28 and @b not Jan 31 we started
 with!

 So, unless you plan to implement some kind of natural language parsing in the
 program, you should probably use wxTimeSpan instead of wxDateSpan (which is
 also more efficient). However, wxDateSpan may be very useful in situations
 when you do need to understand what 'in a month' means (of course, it is
 just @c wxDateTime::Now() + wxDateSpan::Month()).



 @section overview_datetime_arithmetics Date arithmetics

 Many different operations may be performed with the dates, however not all of
 them make sense. For example, multiplying a date by a number is an invalid
 operation, even though multiplying either of the time span classes by a number
 is perfectly valid.

 Here is what can be done:

 @li @b Addition: a wxTimeSpan or wxDateSpan can be added to wxDateTime
     resulting in a new wxDateTime object and also 2 objects of the same span class
     can be added together giving another object of the same class.

 @li @b Subtraction: the same types of operations as above are
     allowed and, additionally, a difference between two wxDateTime objects can be
     taken and this will yield wxTimeSpan.

 @li @b Multiplication: a wxTimeSpan or wxDateSpan object can be
     multiplied by an integer number resulting in an object of the same type.

 @li <b>Unary minus</b>: a wxTimeSpan or wxDateSpan object may finally be
     negated giving an interval of the same magnitude but of opposite time
     direction.

 For all these operations there are corresponding global (overloaded) operators
 and also member functions which are synonyms for them: Add(), Subtract() and
 Multiply(). Unary minus as well as composite assignment operations (like +=)
 are only implemented as members and Neg() is the synonym for unary minus.



 @section overview_datetime_timezones Time zone considerations

 Although the time is always stored internally in GMT, you will usually work in
 the local time zone. Because of this, all wxDateTime constructors and setters
 which take the broken down date assume that these values are for the local
 time zone. Thus, @c wxDateTime(1, wxDateTime::Jan, 1970) will not
 correspond to the wxDateTime Epoch unless you happen to live in the UK.
 All methods returning the date components (year, month, day, hour, minute,
 second...) will also return the correct values for the local time zone by
 default, so, generally, doing the natural things will lead to natural and
 correct results.

 If you only want to do this, you may safely skip the rest of this section.
 However, if you want to work with different time zones, you should read it to
 the end.

 In this (rare) case, you are still limited to the local time zone when
 constructing wxDateTime objects, i.e. there is no way to construct a
 wxDateTime corresponding to the given date in, say, Pacific Standard Time.
 To do it, you will need to call #ToTimezone or #MakeTimezone methods to adjust 
 the date for the target time zone. There are also special versions of these functions
 #ToUTC and #MakeUTC for the most common case - when the date should be constructed in UTC.

 You also can just retrieve the value for some time zone without converting the
 object to it first. For this you may pass TimeZone argument to any of the
 methods which are affected by the time zone (all methods getting date
 components and the date formatting ones, for example). In particular, the
 Format() family of methods accepts a TimeZone parameter and this allows to
 simply print time in any time zone.

 To see how to do it, the last issue to address is how to construct a TimeZone
 object which must be passed to all these methods. First of all, you may construct
 it manually by specifying the time zone offset in seconds from GMT, but
 usually you will just use one of the @ref overview_datetime and
 let the conversion constructor do the job.

 I.e. you would just write

 @code
 wxDateTime dt(...whatever...);
 printf("The time is %s in local time zone", dt.FormatTime().c_str());
 printf("The time is %s in GMT", dt.FormatTime(wxDateTime::GMT).c_str());
 @endcode



 @section overview_datetime_dst Daylight saving time (DST)

 DST (a.k.a. 'summer time') handling is always a delicate task which is better
 left to the operating system which is supposed to be configured by the
 administrator to behave correctly. Unfortunately, when doing calculations with
 date outside of the range supported by the standard library, we are forced to
 deal with these issues ourselves.

 Several functions are provided to calculate the beginning and end of DST in
 the given year and to determine whether it is in effect at the given moment or
 not, but they should not be considered as absolutely correct because, first of
 all, they only work more or less correctly for only a handful of countries
 (any information about other ones appreciated!) and even for them the rules
 may perfectly well change in the future.

 The time zone handling #methods use these functions
 too, so they are subject to the same limitations.



 @section overview_datetime_holidays wxDateTime and Holidays

 TODO.



 @section overview_datetime_compat Compatibility

 The old classes for date/time manipulations ported from wxWidgets version 1.xx
 are still included but are reimplemented in terms of wxDateTime. However, using
 them is strongly discouraged because they have a few quirks/bugs and were not
 'Y2K' compatible.

*/