Qwt User's Guide
QwtDate Class Reference

A collection of methods around date/time values. More...

#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".

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.

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 

"The week with January 1.1 in it."

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:
dateTimeDatetime value
intervalTypeInterval 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:
yearYear
typeOption 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:
dateTimeDatetime value
intervalTypeInterval 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:
valueNumber of milliseconds since the epoch, 1970-01-01T00:00:00 UTC
timeSpecTime 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:
dateTimeDatetime 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:
dateTimeDatetime value
formatFormat string
week0TypeSpecification 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:
dateTimeDatetime value
Returns:
Offset in seconds
int QwtDate::weekNumber ( const QDate &  date,
Week0Type  type 
) [static]

Find the week number of a date

Parameters:
dateDate
typeOption how to identify the first week
Returns:
Week number, starting with 1

List of all members.

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