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[wxWidgets.git] / docs / doxygen / overviews / datetime.h
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15b6757b 1/////////////////////////////////////////////////////////////////////////////
d54cf7ff 2// Name: datetime.h
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3// Purpose: topic overview
4// Author: wxWidgets team
5// RCS-ID: $Id$
6// Licence: wxWindows license
7/////////////////////////////////////////////////////////////////////////////
8
880efa2a 9/**
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928f1a07 11@page overview_datetime Date and Time
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928f1a07 13Classes: wxDateTime, wxDateSpan, wxTimeSpan, wxCalendarCtrl
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15@li @ref overview_datetime_introduction
16@li @ref overview_datetime_classes
17@li @ref overview_datetime_characteristics
18@li @ref overview_datetime_timespandiff
19@li @ref overview_datetime_arithmetics
20@li @ref overview_datetime_timezones
21@li @ref overview_datetime_dst
22@li @ref overview_datetime_holidays
23@li @ref overview_datetime_compat
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24
25
928f1a07 26<hr>
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928f1a07 29@section overview_datetime_introduction Introduction
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31wxWidgets provides a set of powerful classes to work with dates and times. Some
32of the supported features of wxDateTime class are:
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34@li Wide range: the range of supported dates goes from about 4714 B.C. to
35 some 480 million years in the future.
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37@li Precision: not using floating point calculations anywhere ensures that
38 the date calculations don't suffer from rounding errors.
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40@li Many features: not only all usual calculations with dates are supported,
41 but also more exotic week and year day calculations, work day testing, standard
42 astronomical functions, conversion to and from strings in either strict or free
d54cf7ff 43 format.
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45@li Efficiency: objects of wxDateTime are small (8 bytes) and working with
46 them is fast
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47
48
49
928f1a07 50@section overview_datetime_classes All date/time classes at a glance
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52There are 3 main classes declared in @c wx/datetime.h: except wxDateTime itself
53which represents an absolute moment in time, there are also two classes -
54wxTimeSpan and wxDateSpan - which represent the intervals of time.
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56There are also helper classes which are used together with wxDateTime:
57wxDateTimeHolidayAuthority which is used to determine whether a given date
58is a holiday or not and wxDateTimeWorkDays which is a derivation of this
59class for which (only) Saturdays and Sundays are the holidays. See more about
60these classes in the discussion of the holidays (see @ref overview_datetime_holidays).
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62Finally, in other parts of this manual you may find mentions of wxDate and
63wxTime classes. @ref overview_datetime_compat are obsolete and
64superseded by wxDateTime.
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66
67
928f1a07 68@section overview_datetime_characteristics wxDateTime characteristics
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70wxDateTime stores the time as a signed number of
71milliseconds since the Epoch which is fixed, by convention, to Jan 1, 1970 -
72however this is not visible to the class users (in particular, dates prior to
73the Epoch are handled just as well (or as bad) as the dates after it). But it
74does mean that the best resolution which can be achieved with this class is 1
75millisecond.
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77The size of wxDateTime object is 8 bytes because it is represented as a 64 bit
78integer. The resulting range of supported dates is thus approximatively 580
79million years, but due to the current limitations in the Gregorian calendar
80support, only dates from Nov 24, 4714BC are supported (this is subject to
81change if there is sufficient interest in doing it).
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83Finally, the internal representation is time zone independent (always in GMT)
84and the time zones only come into play when a date is broken into
85year/month/day components. See more about timezones below
86(see @ref overview_datetime_timezones).
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88Currently, the only supported calendar is Gregorian one (which is used even
89for the dates prior to the historic introduction of this calendar which was
90first done on Oct 15, 1582 but is, generally speaking, country, and even
91region, dependent). Future versions will probably have Julian calendar support
92as well and support for other calendars (Maya, Hebrew, Chinese...) is not
93ruled out.
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95
96
928f1a07 97@section overview_datetime_timespandiff Difference between wxDateSpan and wxTimeSpan
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99While there is only one logical way to represent an absolute moment in the
100time (and hence only one wxDateTime class), there are at least two methods to
101describe a time interval.
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103First, there is the direct and self-explaining way implemented by
104wxTimeSpan: it is just a difference in milliseconds
105between two moments in time. Adding or subtracting such an interval to
106wxDateTime is always well-defined and is a fast operation.
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108But in the daily life other, calendar-dependent time interval specifications are
109used. For example, 'one month later' is commonly used. However, it is clear
110that this is not the same as wxTimeSpan of 60*60*24*31 seconds because 'one
111month later' Feb 15 is Mar 15 and not Mar 17 or Mar 16 (depending on whether
112the year is leap or not).
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114This is why there is another class for representing such intervals called
115wxDateSpan. It handles these sort of operations in the
116most natural way possible, but note that manipulating with intervals of
117this kind is not always well-defined. Consider, for example, Jan 31 + '1
118month': this will give Feb 28 (or 29), i.e. the last day of February and not
119the non-existent Feb 31. Of course, this is what is usually wanted, but you
120still might be surprised to notice that now subtracting back the same
121interval from Feb 28 will result in Jan 28 and @b not Jan 31 we started
122with!
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124So, unless you plan to implement some kind of natural language parsing in the
125program, you should probably use wxTimeSpan instead of wxDateSpan (which is
126also more efficient). However, wxDateSpan may be very useful in situations
127when you do need to understand what 'in a month' means (of course, it is
128just @c wxDateTime::Now() + wxDateSpan::Month()).
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130
131
928f1a07 132@section overview_datetime_arithmetics Date arithmetics
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134Many different operations may be performed with the dates, however not all of
135them make sense. For example, multiplying a date by a number is an invalid
136operation, even though multiplying either of the time span classes by a number
137is perfectly valid.
36c9828f 138
928f1a07 139Here is what can be done:
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141@li @b Addition: a wxTimeSpan or wxDateSpan can be added to wxDateTime
142 resulting in a new wxDateTime object and also 2 objects of the same span class
143 can be added together giving another object of the same class.
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145@li @b Subtraction: the same types of operations as above are
146 allowed and, additionally, a difference between two wxDateTime objects can be
147 taken and this will yield wxTimeSpan.
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149@li @b Multiplication: a wxTimeSpan or wxDateSpan object can be
150 multiplied by an integer number resulting in an object of the same type.
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152@li <b>Unary minus</b>: a wxTimeSpan or wxDateSpan object may finally be
153 negated giving an interval of the same magnitude but of opposite time
154 direction.
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156For all these operations there are corresponding global (overloaded) operators
157and also member functions which are synonyms for them: Add(), Subtract() and
158Multiply(). Unary minus as well as composite assignment operations (like +=)
159are only implemented as members and Neg() is the synonym for unary minus.
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161
162
928f1a07 163@section overview_datetime_timezones Time zone considerations
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165Although the time is always stored internally in GMT, you will usually work in
166the local time zone. Because of this, all wxDateTime constructors and setters
167which take the broken down date assume that these values are for the local
168time zone. Thus, @c wxDateTime(1, wxDateTime::Jan, 1970) will not
169correspond to the wxDateTime Epoch unless you happen to live in the UK.
170All methods returning the date components (year, month, day, hour, minute,
171second...) will also return the correct values for the local time zone by
172default, so, generally, doing the natural things will lead to natural and
173correct results.
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175If you only want to do this, you may safely skip the rest of this section.
176However, if you want to work with different time zones, you should read it to
177the end.
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179In this (rare) case, you are still limited to the local time zone when
180constructing wxDateTime objects, i.e. there is no way to construct a
181wxDateTime corresponding to the given date in, say, Pacific Standard Time.
182To do it, you will need to call wxDateTime::ToTimezone or wxDateTime::MakeTimezone
183methods to adjust the date for the target time zone. There are also special
184versions of these functions wxDateTime::ToUTC and wxDateTime::MakeUTC for
185the most common case - when the date should be constructed in UTC.
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187You also can just retrieve the value for some time zone without converting the
188object to it first. For this you may pass TimeZone argument to any of the
189methods which are affected by the time zone (all methods getting date
190components and the date formatting ones, for example). In particular, the
191Format() family of methods accepts a TimeZone parameter and this allows to
192simply print time in any time zone.
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194To see how to do it, the last issue to address is how to construct a TimeZone
195object which must be passed to all these methods. First of all, you may construct
196it manually by specifying the time zone offset in seconds from GMT, but
197usually you will just use one of the @ref overview_datetime and
198let the conversion constructor do the job.
d54cf7ff 199
928f1a07 200I.e. you would just write
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202@code
203wxDateTime dt(...whatever...);
204printf("The time is %s in local time zone", dt.FormatTime().c_str());
205printf("The time is %s in GMT", dt.FormatTime(wxDateTime::GMT).c_str());
206@endcode
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207
208
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928f1a07 210@section overview_datetime_dst Daylight saving time (DST)
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212DST (a.k.a. 'summer time') handling is always a delicate task which is better
213left to the operating system which is supposed to be configured by the
214administrator to behave correctly. Unfortunately, when doing calculations with
215date outside of the range supported by the standard library, we are forced to
216deal with these issues ourselves.
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218Several functions are provided to calculate the beginning and end of DST in
219the given year and to determine whether it is in effect at the given moment or
220not, but they should not be considered as absolutely correct because, first of
221all, they only work more or less correctly for only a handful of countries
222(any information about other ones appreciated!) and even for them the rules
223may perfectly well change in the future.
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225The time zone handling methods (see @ref overview_datetime_timezones) use
226these functions too, so they are subject to the same limitations.
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228
229
928f1a07 230@section overview_datetime_holidays wxDateTime and Holidays
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928f1a07 232@todo WRITE THIS DOC PARAGRAPH.
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234
235
928f1a07 236@section overview_datetime_compat Compatibility
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238The old classes for date/time manipulations ported from wxWidgets version 1.xx
239are still included but are reimplemented in terms of wxDateTime. However, using
240them is strongly discouraged because they have a few quirks/bugs and were not
241'Y2K' compatible.
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d54cf7ff 243*/
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