36c9828f70
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@51911 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
298 lines
10 KiB
C++
298 lines
10 KiB
C++
/////////////////////////////////////////////////////////////////////////////
|
|
// Name: datetime
|
|
// Purpose: topic overview
|
|
// Author: wxWidgets team
|
|
// RCS-ID: $Id$
|
|
// Licence: wxWindows license
|
|
/////////////////////////////////////////////////////////////////////////////
|
|
|
|
/*!
|
|
|
|
@page datetime_overview Date and time classes overview
|
|
|
|
Classes: #wxDateTime, #wxDateSpan, #wxTimeSpan, #wxCalendarCtrl
|
|
#Introduction
|
|
@ref alldatetimeclasses_overview
|
|
@ref datetimecharacteristics_overview
|
|
@ref dateandtimespansdifference_overview
|
|
@ref tdatearithm_overview
|
|
@ref tdatetimezones_overview
|
|
#Daylight saving time (DST)
|
|
@ref tdateholidays_overview
|
|
#Compatibility
|
|
|
|
|
|
@section introductiontowxdatetime Introduction
|
|
|
|
wxWidgets provides a set of powerful classes to work with dates and times. Some
|
|
of the supported features of #wxDateTime class are:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Wide range
|
|
|
|
|
|
|
|
|
|
The range of supported dates goes from about 4714 B.C. to
|
|
some 480 million years in the future.
|
|
|
|
|
|
|
|
|
|
|
|
Precision
|
|
|
|
|
|
|
|
|
|
Not using floating point calculations anywhere ensures that
|
|
the date calculations don't suffer from rounding errors.
|
|
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
|
|
Efficiency
|
|
|
|
|
|
|
|
|
|
Objects of wxDateTime are small (8 bytes) and working with
|
|
them is fast
|
|
|
|
|
|
|
|
|
|
|
|
|
|
@section alldatetimeclasses 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 tdatecompatibility_overview are obsolete and
|
|
superseded by wxDateTime.
|
|
|
|
@section wxdatetimecharacteristics 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 dateandtimespansdifference 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 tdatearithm 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:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
@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.
|
|
|
|
|
|
|
|
|
|
|
|
@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.
|
|
|
|
|
|
|
|
|
|
|
|
@b Multiplication
|
|
|
|
|
|
|
|
|
|
a wxTimeSpan or wxDateSpan object can be
|
|
multiplied by an integer number resulting in an object of the same type.
|
|
|
|
|
|
|
|
|
|
|
|
@b Unary minus
|
|
|
|
|
|
|
|
|
|
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 tdatetimezones 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 datetime_overview 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 tdatedst 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 tdateholidays wxDateTime and Holidays
|
|
|
|
TODO.
|
|
|
|
@section tdatecompatibility 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.
|
|
|
|
*/
|
|
|
|
|