QwtDate(3) Qwt User's Guide QwtDate(3)

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

#include <qwt_date.h>


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


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)

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

Definition at line 42 of file qwt_date.h.

Enumerator

The Julian day of 'The Epoch'.

Definition at line 102 of file qwt_date.h.

enum QwtDate::IntervalType

Classification of an time interval

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

Enumerator

The interval is related to milliseconds.
The interval is related to seconds.
The interval is related to minutes.
The interval is related to hours.
The interval is related to days.
The interval is related to weeks.
The interval is related to months.
The interval is related to years.

Definition at line 75 of file qwt_date.h.

enum QwtDate::Week0Type

How to identify the first week of year differs between countries.

Enumerator

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().

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

Definition at line 49 of file qwt_date.h.

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()

Definition at line 323 of file qwt_date.cpp.

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()

Definition at line 542 of file qwt_date.cpp.

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()

Definition at line 425 of file qwt_date.cpp.

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

Definition at line 521 of file qwt_date.cpp.

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()

Definition at line 499 of file qwt_date.cpp.

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

Definition at line 261 of file qwt_date.cpp.

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.

Definition at line 298 of file qwt_date.cpp.

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

Definition at line 686 of file qwt_date.cpp.

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.offsetFromUtc()
  • 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

Definition at line 635 of file qwt_date.cpp.

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

Definition at line 585 of file qwt_date.cpp.

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Sun Jul 18 2021 Version 6.2.0