docs/doxygen/overviews/datetime.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        datetime.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows licence
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /**
  10
  11 @page overview_datetime Date and Time
  12
  13 Classes: wxDateTime, wxDateSpan, wxTimeSpan, wxCalendarCtrl
  14
  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
  24
  25
  26 <hr>
  27
  28
  29 @section overview_datetime_introduction Introduction
  30
  31 wxWidgets provides a set of powerful classes to work with dates and times. Some
  32 of the supported features of wxDateTime class are:
  33
  34 @li Wide range: the range of supported dates goes from about 4714 B.C. to
  35     some 480 million years in the future.
  36
  37 @li Precision: not using floating point calculations anywhere ensures that
  38     the date calculations don't suffer from rounding errors.
  39
  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
  43     format.
  44
  45 @li Efficiency: objects of wxDateTime are small (8 bytes) and working with
  46     them is fast
  47
  48
  49
  50 @section overview_datetime_classes All date/time classes at a glance
  51
  52 There are 3 main classes declared in @c wx/datetime.h: except wxDateTime itself
  53 which represents an absolute moment in time, there are also two classes -
  54 wxTimeSpan and wxDateSpan - which represent the intervals of time.
  55
  56 There are also helper classes which are used together with wxDateTime:
  57 wxDateTimeHolidayAuthority which is used to determine whether a given date
  58 is a holiday or not and wxDateTimeWorkDays which is a derivation of this
  59 class for which (only) Saturdays and Sundays are the holidays. See more about
  60 these classes in the discussion of the holidays (see @ref overview_datetime_holidays).
  61
  62 Finally, in other parts of this manual you may find mentions of wxDate and
  63 wxTime classes. @ref overview_datetime_compat are obsolete and
  64 superseded by wxDateTime.
  65
  66
  67
  68 @section overview_datetime_characteristics wxDateTime characteristics
  69
  70 wxDateTime stores the time as a signed number of
  71 milliseconds since the Epoch which is fixed, by convention, to Jan 1, 1970 -
  72 however this is not visible to the class users (in particular, dates prior to
  73 the Epoch are handled just as well (or as bad) as the dates after it). But it
  74 does mean that the best resolution which can be achieved with this class is 1
  75 millisecond.
  76
  77 The size of wxDateTime object is 8 bytes because it is represented as a 64 bit
  78 integer. The resulting range of supported dates is thus approximatively 580
  79 million years, but due to the current limitations in the Gregorian calendar
  80 support, only dates from Nov 24, 4714BC are supported (this is subject to
  81 change if there is sufficient interest in doing it).
  82
  83 Finally, the internal representation is time zone independent (always in GMT)
  84 and the time zones only come into play when a date is broken into
  85 year/month/day components. See more about timezones below
  86 (see @ref overview_datetime_timezones).
  87
  88 Currently, the only supported calendar is Gregorian one (which is used even
  89 for the dates prior to the historic introduction of this calendar which was
  90 first done on Oct 15, 1582 but is, generally speaking, country, and even
  91 region, dependent). Future versions will probably have Julian calendar support
  92 as well and support for other calendars (Maya, Hebrew, Chinese...) is not
  93 ruled out.
  94
  95
  96
  97 @section overview_datetime_timespandiff Difference between wxDateSpan and wxTimeSpan
  98
  99 While there is only one logical way to represent an absolute moment in the
 100 time (and hence only one wxDateTime class), there are at least two methods to
 101 describe a time interval.
 102
 103 First, there is the direct and self-explaining way implemented by
 104 wxTimeSpan: it is just a difference in milliseconds
 105 between two moments in time. Adding or subtracting such an interval to
 106 wxDateTime is always well-defined and is a fast operation.
 107
 108 But in the daily life other, calendar-dependent time interval specifications are
 109 used. For example, 'one month later' is commonly used. However, it is clear
 110 that this is not the same as wxTimeSpan of 60*60*24*31 seconds because 'one
 111 month later' Feb 15 is Mar 15 and not Mar 17 or Mar 16 (depending on whether
 112 the year is leap or not).
 113
 114 This is why there is another class for representing such intervals called
 115 wxDateSpan. It handles these sort of operations in the
 116 most natural way possible, but note that manipulating with intervals of
 117 this kind is not always well-defined. Consider, for example, Jan 31 + '1
 118 month': this will give Feb 28 (or 29), i.e. the last day of February and not
 119 the non-existent Feb 31. Of course, this is what is usually wanted, but you
 120 still might be surprised to notice that now subtracting back the same
 121 interval from Feb 28 will result in Jan 28 and @b not Jan 31 we started
 122 with!
 123
 124 So, unless you plan to implement some kind of natural language parsing in the
 125 program, you should probably use wxTimeSpan instead of wxDateSpan (which is
 126 also more efficient). However, wxDateSpan may be very useful in situations
 127 when you do need to understand what 'in a month' means (of course, it is
 128 just @c wxDateTime::Now() + wxDateSpan::Month()).
 129
 130
 131
 132 @section overview_datetime_arithmetics Date arithmetics
 133
 134 Many different operations may be performed with the dates, however not all of
 135 them make sense. For example, multiplying a date by a number is an invalid
 136 operation, even though multiplying either of the time span classes by a number
 137 is perfectly valid.
 138
 139 Here is what can be done:
 140
 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.
 144
 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.
 148
 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.
 151
 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.
 155
 156 For all these operations there are corresponding global (overloaded) operators
 157 and also member functions which are synonyms for them: Add(), Subtract() and
 158 Multiply(). Unary minus as well as composite assignment operations (like +=)
 159 are only implemented as members and Neg() is the synonym for unary minus.
 160
 161
 162
 163 @section overview_datetime_timezones Time zone considerations
 164
 165 Although the time is always stored internally in GMT, you will usually work in
 166 the local time zone. Because of this, all wxDateTime constructors and setters
 167 which take the broken down date assume that these values are for the local
 168 time zone. Thus, @c wxDateTime(1, wxDateTime::Jan, 1970) will not
 169 correspond to the wxDateTime Epoch unless you happen to live in the UK.
 170 All methods returning the date components (year, month, day, hour, minute,
 171 second...) will also return the correct values for the local time zone by
 172 default, so, generally, doing the natural things will lead to natural and
 173 correct results.
 174
 175 If you only want to do this, you may safely skip the rest of this section.
 176 However, if you want to work with different time zones, you should read it to
 177 the end.
 178
 179 In this (rare) case, you are still limited to the local time zone when
 180 constructing wxDateTime objects, i.e. there is no way to construct a
 181 wxDateTime corresponding to the given date in, say, Pacific Standard Time.
 182 To do it, you will need to call wxDateTime::ToTimezone or wxDateTime::MakeTimezone
 183 methods to adjust the date for the target time zone. There are also special
 184 versions of these functions wxDateTime::ToUTC and wxDateTime::MakeUTC for
 185 the most common case - when the date should be constructed in UTC.
 186
 187 You also can just retrieve the value for some time zone without converting the
 188 object to it first. For this you may pass TimeZone argument to any of the
 189 methods which are affected by the time zone (all methods getting date
 190 components and the date formatting ones, for example). In particular, the
 191 Format() family of methods accepts a TimeZone parameter and this allows to
 192 simply print time in any time zone.
 193
 194 To see how to do it, the last issue to address is how to construct a TimeZone
 195 object which must be passed to all these methods. First of all, you may construct
 196 it manually by specifying the time zone offset in seconds from GMT, but
 197 usually you will just use one of the @ref overview_datetime and
 198 let the conversion constructor do the job.
 199
 200 I.e. you would just write
 201
 202 @code
 203 wxDateTime dt(...whatever...);
 204 printf("The time is %s in local time zone", dt.FormatTime().c_str());
 205 printf("The time is %s in GMT", dt.FormatTime(wxDateTime::GMT).c_str());
 206 @endcode
 207
 208
 209
 210 @section overview_datetime_dst Daylight saving time (DST)
 211
 212 DST (a.k.a. 'summer time') handling is always a delicate task which is better
 213 left to the operating system which is supposed to be configured by the
 214 administrator to behave correctly. Unfortunately, when doing calculations with
 215 date outside of the range supported by the standard library, we are forced to
 216 deal with these issues ourselves.
 217
 218 Several functions are provided to calculate the beginning and end of DST in
 219 the given year and to determine whether it is in effect at the given moment or
 220 not, but they should not be considered as absolutely correct because, first of
 221 all, 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
 223 may perfectly well change in the future.
 224
 225 The time zone handling methods (see @ref overview_datetime_timezones) use
 226 these functions too, so they are subject to the same limitations.
 227
 228
 229
 230 @section overview_datetime_holidays wxDateTime and Holidays
 231
 232 @todo WRITE THIS DOC PARAGRAPH.
 233
 234
 235
 236 @section overview_datetime_compat Compatibility
 237
 238 The old classes for date/time manipulations ported from wxWidgets version 1.xx
 239 are still included but are reimplemented in terms of wxDateTime. However, using
 240 them is strongly discouraged because they have a few quirks/bugs and were not
 241 'Y2K' compatible.
 242
 243 */
 244