QwtDate(3)

NAME
QwtDate - A collection of methods around date/time values.

SYNOPSIS

#include <qwt_date.h>

  Public Types
enum Week0Type { FirstThursday, FirstDay }
enum IntervalType { Millisecond, Second, Minute, Hour, Day, Week, Month, Year }
enum { JulianDayForEpoch = 2440588 }

  Static Public Member Functions
static QDate minDate ()
static QDate maxDate ()
static QDateTime toDateTime (double value, Qt::TimeSpec=Qt::UTC)
static double toDouble (const QDateTime &)
static QDateTime ceil (const QDateTime &, IntervalType)
static QDateTime floor (const QDateTime &, IntervalType)
static QDate dateOfWeek0 (int year, Week0Type)
Date of the first day of the first week for a year.
static int weekNumber (const QDate &, Week0Type)
static int utcOffset (const QDateTime &)
static QString toString (const QDateTime &, const QString &format, Week0Type)

Detailed Description

A collection of methods around date/time values. Qt offers convenient classes for dealing with date/time values, but Qwt uses coordinate systems that are based on doubles. QwtDate offers methods to translate from QDateTime to double and v.v.

A double is interpreted as the number of milliseconds since 1970-01-01T00:00:00 Universal Coordinated Time - also known as 'The Epoch'.

While the range of the Julian day in Qt4 is limited to [0, MAX_INT], Qt5 stores it as qint64 offering a huge range of valid dates. As the significance of a double is below this ( assuming a fraction of 52 bits ) the translation is not bijective with rounding errors for dates very far from Epoch. For a resolution of 1 ms those start to happen for dates above the year 144683.

An axis for a date/time interval is expected to be aligned and divided in time/date units like seconds, minutes, ... QwtDate offers several algorithms that are needed to calculate these axes.

See also
    QwtDateScaleEngine, QwtDateScaleDraw, QDate, QTime

Member Enumeration Documentation

  anonymous enum

Enumerator

JulianDayForEpoch
    The Julian day of 'The Epoch'.

  enum QwtDate::IntervalType
Classification of an time interval

Time intervals needs to be classified to decide how to align and divide it.

Enumerator

Millisecond
    The interval is related to milliseconds.

Second
    The interval is related to seconds.

Minute
    The interval is related to minutes.

Hour
    The interval is related to hours.

Day The interval is related to days.

Week
    The interval is related to weeks.

Month
    The interval is related to months.

Year
    The interval is related to years.

  enum QwtDate::Week0Type
How to identify the first week of year differs between countries.

Enumerator

FirstThursday
    According to ISO 8601 the first week of a year is defined as 'the week with the year's first Thursday in it'.

FirstThursday corresponds to the numbering that is implemented in QDate::weekNumber().

FirstDay

In the U.S. this definition is more common than FirstThursday.

Member Function Documentation

  QDateTime QwtDate::ceil (const QDateTime & dateTime, IntervalType intervalType) [static]
Ceil a datetime according the interval type

Parameters
    dateTime Datetime value
    intervalType Interval type, how to ceil. F.e. when intervalType = QwtDate::Months, the result will be ceiled to the next beginning of a month

Returns
    Ceiled datetime

See also
    floor()

  QDate QwtDate::dateOfWeek0 (int year, Week0Type type) [static]

Date of the first day of the first week for a year. The first day of a week depends on the current locale ( QLocale::firstDayOfWeek() ).

Parameters
    year Year
    type Option how to identify the first week

Returns
    First day of week 0

See also
    QLocale::firstDayOfWeek(), weekNumber()

  QDateTime QwtDate::floor (const QDateTime & dateTime, IntervalType intervalType) [static]
Floor a datetime according the interval type

Parameters
    dateTime Datetime value
    intervalType Interval type, how to ceil. F.e. when intervalType = QwtDate::Months, the result will be ceiled to the next beginning of a month

Returns
    Floored datetime

See also
    floor()

  QDate QwtDate::maxDate () [static]
Maximum for the supported date range

The range of valid dates depends on how QDate stores the Julian day internally.

•   For Qt4 it is 'Tue Jun 3 5874898'

•   For Qt5 it is 'Tue Dec 31 2147483647'

Returns
    maximum of the date range

See also
    minDate()

Note
    The maximum differs between Qt4 and Qt5

  QDate QwtDate::minDate () [static]
Minimum for the supported date range

The range of valid dates depends on how QDate stores the Julian day internally.

•   For Qt4 it is 'Tue Jan 2 -4713'

•   For Qt5 it is 'Thu Jan 1 -2147483648'

Returns
    minimum of the date range

See also
    maxDate()

  QDateTime QwtDate::toDateTime (double value, Qt::TimeSpec timeSpec = Qt::UTC) [static]
Translate from double to QDateTime

Parameters
    value Number of milliseconds since the epoch, 1970-01-01T00:00:00 UTC
    timeSpec Time specification

Returns
    Datetime value

See also
    toDouble(), QDateTime::setMSecsSinceEpoch()

Note
    The return datetime for Qt::OffsetFromUTC will be Qt::UTC

  double QwtDate::toDouble (const QDateTime & dateTime) [static]
Translate from QDateTime to double

Parameters
    dateTime Datetime value

Returns
    Number of milliseconds since 1970-01-01T00:00:00 UTC has passed.

See also
    toDateTime(), QDateTime::toMSecsSinceEpoch()

Warning
    For values very far below or above 1970-01-01 UTC rounding errors will happen due to the limited significance of a double.

  QString QwtDate::toString (const QDateTime & dateTime, const QString & format, Week0Type week0Type) [static]
Translate a datetime into a string

Beside the format expressions documented in QDateTime::toString() the following expressions are supported:

•   w
     week number: ( 1 - 53 )

•   ww
     week number with a leading zero ( 01 - 53 )

As week 1 usually starts in the previous year a special rule is applied for formats, where the year is expected to match the week number - even if the date belongs to the previous year.

Parameters
    dateTime Datetime value
    format Format string
    week0Type Specification of week 0

Returns
    Datetime string

See also
    QDateTime::toString(), weekNumber(), QwtDateScaleDraw

  int QwtDate::utcOffset (const QDateTime & dateTime) [static]
Offset in seconds from Coordinated Universal Time

The offset depends on the time specification of dateTime:

•   Qt::UTC 0, dateTime has no offset

•   Qt::OffsetFromUTC returns dateTime.utcOffset()

•   Qt::LocalTime: number of seconds from the UTC

For Qt::LocalTime the offset depends on the timezone and daylight savings.

Parameters
    dateTime Datetime value

Returns
    Offset in seconds

  int QwtDate::weekNumber (const QDate & date, Week0Type type) [static]
Find the week number of a date

•   QwtDate::FirstThursday
     Corresponding to ISO 8601 ( see QDate::weekNumber() ).

•   QwtDate::FirstDay
     Number of weeks that have begun since dateOfWeek0().

Parameters
    date Date
    type Option how to identify the first week

Returns
    Week number, starting with 1

Author

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