docs/latex/wx/tdate.tex

   1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   2 %% Name:        tdate.tex
   3 %% Purpose:     wxDateTime and related classes overview
   4 %% Author:      Vadim Zeitlin
   5 %% Modified by:
   6 %% Created:     07.03.00
   7 %% RCS-ID:      $Id$
   8 %% Copyright:   (c) Vadim Zeitlin
   9 %% License:     wxWindows license
  10 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  11
  12 \section{Date and time classes overview}\label{wxdatetimeoverview}
  13
  14 Classes: \helpref{wxDateTime}{wxdatetime}, \helpref{wxDateSpan}{wxdatespan}, \helpref{wxTimeSpan}{wxtimespan}, \helpref{wxCalendarCtrl}{wxcalendarctrl}
  15
  16 \subsection{Introduction}
  17
  18 wxWindows provides a set of powerful classes to work with dates and times. Some
  19 of the supported features of \helpref{wxDateTime}{wxdatetime} class are:
  20
  21 \twocolwidtha{7cm}
  22 \begin{twocollist}\itemsep=0pt
  23 \twocolitem{Wide range}{The range of supported dates goes from about 4714 B.C. to
  24 some 480 million years in the future.}
  25 \twocolitem{Precision}{Not using floating point calculations anywhere ensures that
  26 the date calculations don't suffer from rounding errors.}
  27 \twocolitem{Many features}{Not only all usual calculations with dates are supported,
  28 but also more exotic week and year day calculations, work day testing, standard
  29 astronomical functions, conversion to and from strings in either strict or free
  30 format.}
  31 \twocolitem{Efficiency}{Objects of wxDateTime are small (8 bytes) and working with
  32 them is fast}
  33 \end{twocollist}
  34
  35 \subsection{All date/time classes at a glance}
  36
  37 There are 3 main classes declared in {\tt <wx/datetime.h>}: except
  38 \helpref{wxDateTime}{wxdatetime} itself which represents an absolute
  39 moment in time, there are also two classes -
  40 \helpref{wxTimeSpan}{wxtimespan} and \helpref{wxDateSpan}{wxdatespan} which
  41 represent the intervals of time.
  42
  43 There are also helper classes which are used together with wxDateTime:
  44 \helpref{wxDateTimeHolidayAuthority}{wxdatetimeholidayauthority} which is used
  45 to determine whether a given date is a holiday or not and
  46 \helpref{wxDateTimeWorkDays}{wxdatetimeworkdays} which is a derivation of this
  47 class for which (only) Saturdays and Sundays are the holidays. See more about
  48 these classes in the discussion of the \helpref{holidays}{tdateholidays}.
  49
  50 Finally, in other parts of this manual you may find mentions of wxDate and
  51 wxTime classes. \helpref{These classes}{tdatecompatibility} are obsolete and
  52 superseded by wxDateTime.
  53
  54 \subsection{wxDateTime characteristics}
  55
  56 \helpref{wxDateTime}{wxdatetime} stores the time as a signed number of
  57 milliseconds since the Epoch which is fixed, by convention, to Jan 1, 1970 -
  58 however this is not visible to the class users (in particular, dates prior to
  59 the Epoch are handled just as well (or as bad) as the dates after it). But it
  60 does mean that the best resolution which can be achieved with this class is 1
  61 millisecond.
  62
  63 The size of wxDateTime object is 8 bytes because it is represented as a 64 bit
  64 integer. The resulting range of supported dates is thus approximatively 580
  65 million years, but due to the current limitations in the Gregorian calendar
  66 support, only dates from Nov 24, 4714BC are supported (this is subject to
  67 change if there is sufficient interest in doing it).
  68
  69 Finally, the internal representation is time zone independent (always in GMT)
  70 and the time zones only come into play when a date is broken into
  71 year/month/day components. See more about \helpref{timezones}{tdatetimezones}
  72 below.
  73
  74 Currently, the only supported calendar is Gregorian one (which is used even
  75 for the dates prior to the historic introduction of this calendar which was
  76 first done on Oct 15, 1582 but is, generally speaking, country, and even
  77 region, dependent). Future versions will probably have Julian calendar support
  78 as well and support for other calendars (Maya, Hebrew, Chinese...) is not
  79 ruled out.
  80
  81 \subsection{Difference between wxDateSpan and wxTimeSpan}
  82
  83 While there is only one logical way to represent an absolute moment in the
  84 time (and hence only one wxDateTime class), there are at least two methods to
  85 describe a time interval.
  86
  87 First, there is the direct and self-explaining way implemented by
  88 \helpref{wxTimeSpan}{wxtimespan}: it is just a difference in milliseconds
  89 between two moments in time. Adding or subtracting such an interval to
  90 wxDateTime is always well-defined and is a fast operation.
  91
  92 But in the daily life other, calendar-dependent time interval specifications are
  93 used. For example, `one month later' is commonly used. However, it is clear
  94 that this is not the same as wxTimeSpan of $60*60*24*31$ seconds because `one
  95 month later' Feb 15 is Mar 15 and not Mar 17 or Mar 16 (depending on whether
  96 the year is leap or not).
  97
  98 This is why there is another class for representing such intervals called
  99 \helpref{wxDateSpan}{wxdatespan}. It handles these sort of operations in the
 100 most natural way possible, but note that manipulating with intervals of
 101 this kind is not always well-defined. Consider, for example, Jan 31 + `1
 102 month': this will give Feb 28 (or 29), i.e. the last day of February and not
 103 the non-existent Feb 31. Of course, this is what is usually wanted, but you
 104 still might be surprised to notice that now subtracting back the same
 105 interval from Feb 28 will result in Jan 28 and {\bf not} Jan 31 we started
 106 with!
 107
 108 So, unless you plan to implement some kind of natural language parsing in the
 109 program, you should probably use wxTimeSpan instead of wxDateSpan (which is
 110 also more efficient). However, wxDateSpan may be very useful in situations
 111 when you do need to understand what `in a month' means (of course, it is
 112 just {\tt wxDateTime::Now() + wxDateSpan::Month()}).
 113
 114 \subsection{Date arithmetics}\label{tdatearithm}
 115
 116 Many different operations may be performed with the dates, however not all of
 117 them make sense. For example, multiplying a date by a number is an invalid
 118 operation, even though multiplying either of the time span classes by a number
 119 is perfectly valid.
 120
 121 Here is what can be done:
 122
 123 \begin{twocollist}\itemsep=0pt
 124 \twocolitem{{\bf Addition}}{a wxTimeSpan or wxDateSpan can be added to wxDateTime
 125 resulting in a new wxDateTime object and also 2 objects of the same span class
 126 can be added together giving another object of the same class.}
 127 \twocolitem{{\bf Subtraction}}{the same types of operations as above are
 128 allowed and, additionally, a difference between two wxDateTime objects can be
 129 taken and this will yield wxTimeSpan.}
 130 \twocolitem{{\bf Multiplication}}{a wxTimeSpan or wxDateSpan object can be
 131 multiplied by an integer number resulting in an object of the same type.}
 132 \twocolitem{{\bf Unary minus}}{a wxTimeSpan or wxDateSpan object may finally be
 133 negated giving an interval of the same magnitude but of opposite time
 134 direction.}
 135 \end{twocollist}
 136
 137 For all these operations there are corresponding global (overloaded) operators
 138 and also member functions which are synonyms for them: Add(), Subtract() and
 139 Multiply(). Unary minus as well as composite assignment operations (like $+=$)
 140 are only implemented as members and Neg() is the synonym for unary minus.
 141
 142 \subsection{Time zone considerations}\label{tdatetimezones}
 143
 144 Although the time is always stored internally in GMT, you will usually work in
 145 the local time zone. Because of this, all wxDateTime constructors and setters
 146 which take the broken down date assume that these values are for the local
 147 time zone. Thus, {\tt wxDateTime(1, wxDateTime::Jan, 1970)} will not
 148 correspond to the wxDateTime Epoch unless you happen to live in the UK.
 149
 150 All methods returning the date components (year, month, day, hour, minute,
 151 second...) will also return the correct values for the local time zone by
 152 default, so, generally, doing the natural things will lead to natural and
 153 correct results.
 154
 155 If you only want to do this, you may safely skip the rest of this section.
 156 However, if you want to work with different time zones, you should read it to
 157 the end.
 158
 159 In this (rare) case, you are still limited to the local time zone when
 160 constructing wxDateTime objects, i.e. there is no way to construct a
 161 wxDateTime corresponding to the given date in, say, Pacific Standard Time.
 162 To do it, you will need to call \helpref{ToTimezone}{wxdatetimetotimezone} or
 163 \helpref{MakeTimezone}{wxdatetimemaketimezone} methods to adjust the date for
 164 the target time zone. There are also special versions of these functions
 165 \helpref{ToGMT}{wxdatetimetogmt} and \helpref{MakeGMT}{wxdatetimemakegmt} for
 166 the most common case - when the date should be constructed in GMT.
 167
 168 You also can just retrieve the value for some time zone without converting the
 169 object to it first. For this you may pass TimeZone argument to any of the
 170 methods which are affected by the time zone (all methods getting date
 171 components and the date formatting ones, for example). In particular, the
 172 Format() family of methods accepts a TimeZone parameter and this allows to
 173 simply print time in any time zone.
 174
 175 To see how to do it, the last issue to address is how to construct a TimeZone
 176 object which must be passed to all these methods. First of all, you may construct
 177 it manually by specifying the time zone offset in seconds from GMT, but
 178 usually you will just use one of the \helpref{symbolic time zone names}{wxdatetime} and
 179 let the conversion constructor do the job.
 180 I.e. you would just write
 181
 182 \begin{verbatim}
 183 wxDateTime dt(...whatever...);
 184 printf("The time is %s in local time zone", dt.FormatTime().c_str());
 185 printf("The time is %s in GMT", dt.FormatTime(wxDateTime::GMT).c_str());
 186 \end{verbatim}
 187
 188 \subsection{Daylight saving time (DST)}\label{tdatedst}
 189
 190 DST (a.k.a. `summer time') handling is always a delicate task which is better
 191 left to the operating system which is supposed to be configured by the
 192 administrator to behave correctly. Unfortunately, when doing calculations with
 193 date outside of the range supported by the standard library, we are forced to
 194 deal with these issues ourselves.
 195
 196 Several functions are provided to calculate the beginning and end of DST in
 197 the given year and to determine whether it is in effect at the given moment or
 198 not, but they should not be considered as absolutely correct because, first of
 199 all, they only work more or less correctly for only a handful of countries
 200 (any information about other ones appreciated!) and even for them the rules
 201 may perfectly well change in the future.
 202
 203 The time zone handling \helpref{methods}{tdatetimezones} use these functions
 204 too, so they are subject to the same limitations.
 205
 206 % is this really needed? \subsection{Conversion to/from text}
 207
 208 \subsection{wxDateTime and Holidays}\label{tdateholidays}
 209
 210 TODO.
 211
 212 \subsection{Compatibility}\label{tdatecompatibility}
 213
 214 The old classes for date/time manipulations ported from wxWindows version 1.xx
 215 are still included but are reimplemented in terms of wxDateTime. However, using
 216 them is strongly discouraged because they have a few quirks/bugs and were not
 217 `Y2K' compatible.
 218