qwt_date_scale_engine.cpp

        // as interval types above hours are not equidistant
        // ( even days might have 23/25 hours because of daylight saving )
        // the stepSize is used as a hint only

        maxMajorSteps = qCeil( ( max - min ) / stepSize );
    }

    const QwtDate::IntervalType intvType = 
        intervalType( from, to, maxMajorSteps );

    QwtScaleDiv scaleDiv;

    if ( intvType == QwtDate::Millisecond )
    {
        // for milliseconds and below we can use the decimal system
        scaleDiv = QwtLinearScaleEngine::divideScale( min, max,
            maxMajorSteps, maxMinorSteps, stepSize );
    }
    else
    {
        const QDateTime minDate = QwtDate::floor( from, intvType );
        const QDateTime maxDate = QwtDate::ceil( to, intvType );

        scaleDiv = buildScaleDiv( minDate, maxDate, 
            maxMajorSteps, maxMinorSteps, intvType );

        // scaleDiv has been calculated from an extended interval
        // adjusted to the step size. We have to shrink it again.

        scaleDiv = scaleDiv.bounded( min, max );
    }

    if ( x1 > x2 )
        scaleDiv.invert();

    return scaleDiv;
}

QwtScaleDiv QwtDateScaleEngine::buildScaleDiv( 
    const QDateTime &minDate, const QDateTime &maxDate,
    int maxMajorSteps, int maxMinorSteps,
    QwtDate::IntervalType intervalType ) const
{
    // calculate the step size
    const double stepSize = qwtDivideScale( 
        qwtIntervalWidth( minDate, maxDate, intervalType ), 
        maxMajorSteps, intervalType );

    // align minDate to the step size
    QDateTime dt0 = alignDate( minDate, stepSize, intervalType, false );
    if ( !dt0.isValid() )
    {
        // the floored date is out of the range of a 
        // QDateTime - we ceil instead.
        dt0 = alignDate( minDate, stepSize, intervalType, true );
    }

    QwtScaleDiv scaleDiv;

    if ( intervalType <= QwtDate::Week )
    {
        scaleDiv = qwtDivideToSeconds( dt0, maxDate, 
            stepSize, maxMinorSteps, intervalType );
    }
    else
    {
        if( intervalType == QwtDate::Month )
        {
            scaleDiv = qwtDivideToMonths( dt0, maxDate,
                stepSize, maxMinorSteps );
        }
        else if ( intervalType == QwtDate::Year )
        {
            scaleDiv = qwtDivideToYears( dt0, maxDate,
                stepSize, maxMinorSteps );
        }
    }


    return scaleDiv;
}

/*!
  Align a date/time value for a step size

  For Qt::Day alignments there is no "natural day 0" -
  instead the first day of the year is used to avoid jumping 
  major ticks positions when panning a scale. For other alignments
  ( f.e according to the first day of the month ) alignDate()
  has to be overloaded.

  \param dateTime Date/time value
  \param stepSize Step size
  \param intervalType Interval type
  \param up When true dateTime is ceiled - otherwise it is floored

  \return Aligned date/time value
 */
QDateTime QwtDateScaleEngine::alignDate( 
    const QDateTime &dateTime, double stepSize, 
    QwtDate::IntervalType intervalType, bool up ) const
{
    // what about: (year == 1582 && month == 10 && day > 4 && day < 15) ??

    QDateTime dt = dateTime;

    if ( dateTime.timeSpec() == Qt::OffsetFromUTC )
    {
        dt.setUtcOffset( 0 );
    }

    switch( intervalType )
    {
        case QwtDate::Millisecond:
        {
            const int ms = qwtAlignValue( 
                dt.time().msec(), stepSize, up ) ;

            dt = QwtDate::floor( dateTime, QwtDate::Second );
            dt = dt.addMSecs( ms );

            break;
        }
        case QwtDate::Second:
        {
            int second = dt.time().second();
            if ( up )
            {
                if ( dt.time().msec() > 0 )
                    second++;
            }

            const int s = qwtAlignValue( second, stepSize, up );

            dt = QwtDate::floor( dt, QwtDate::Minute );
            dt = dt.addSecs( s );

            break;
        }
        case QwtDate::Minute:
        {
            int minute = dt.time().minute();
            if ( up )
            {
                if ( dt.time().msec() > 0 || dt.time().second() > 0 )
                    minute++;
            }

            const int m = qwtAlignValue( minute, stepSize, up );

            dt = QwtDate::floor( dt, QwtDate::Hour );
            dt = dt.addSecs( m * 60 );

            break;
        }
        case QwtDate::Hour:
        {
            int hour = dt.time().hour();
            if ( up )
            {
                if ( dt.time().msec() > 0 || dt.time().second() > 0
                    || dt.time().minute() > 0 )
                {
                    hour++;
                }
            }
            const int h = qwtAlignValue( hour, stepSize, up );

            dt = QwtDate::floor( dt, QwtDate::Day );
            dt = dt.addSecs( h * 3600 );

            break;
        }
        case QwtDate::Day:
        {
            // What date do we expect f.e. from an alignment of 5 days ??
            // Aligning them to the beginning of the year avoids at least
            // jumping major ticks when panning

            int day = dt.date().dayOfYear();
            if ( up )
            {
                if ( dt.time() > QTime( 0, 0 ) )
                    day++;
            }

            const int d = qwtAlignValue( day, stepSize, up );

            dt = QwtDate::floor( dt, QwtDate::Year );
            dt = dt.addDays( d - 1 );

            break;
        }
        case QwtDate::Week:
        {
            const QDate date = QwtDate::dateOfWeek0(
                dt.date().year(), d_data->week0Type );

            int numWeeks = date.daysTo( dt.date() ) / 7;
            if ( up )
            {
                if ( dt.time() > QTime( 0, 0 ) ||
                    date.daysTo( dt.date() ) % 7 )
                {
                    numWeeks++;
                }
            }

            const int d = qwtAlignValue( numWeeks, stepSize, up ) * 7;

            dt = QwtDate::floor( dt, QwtDate::Day );
            dt.setDate( date );
            dt = dt.addDays( d );

            break;
        }
        case QwtDate::Month:
        {
            int month = dt.date().month();
            if ( up )
            {
                if ( dt.date().day() > 1 ||
                    dt.time() > QTime( 0, 0 ) )
                {
                    month++;
                }
            }

            const int m = qwtAlignValue( month - 1, stepSize, up );

            dt = QwtDate::floor( dt, QwtDate::Year );
            dt = dt.addMonths( m );

            break;
        }
        case QwtDate::Year:
        {
            int year = dateTime.date().year();
            if ( up )
            {
                if ( dateTime.date().dayOfYear() > 1 ||
                    dt.time() > QTime( 0, 0 ) )
                {
                    year++;
                }
            }

            const int y = qwtAlignValue( year, stepSize, up );

            dt = QwtDate::floor( dt, QwtDate::Day );
            if ( y == 0 )
            {
                // there is no year 0 in the Julian calendar
                dt.setDate( QDate( stepSize, 1, 1 ).addYears( -stepSize ) );
            }
            else
            {
                dt.setDate( QDate( y, 1, 1 ) );
            }

            break;
        }
    }

    if ( dateTime.timeSpec() == Qt::OffsetFromUTC )
    {
        dt.setUtcOffset( dateTime.utcOffset() );
    }

    return dt;
}

/*!
  Translate a double value into a QDateTime object.

  For QDateTime result is bounded by QwtDate::minDate() and QwtDate::maxDate()

  \return QDateTime object initialized with timeSpec() and utcOffset().
  \sa timeSpec(), utcOffset(), QwtDate::toDateTime()
 */
QDateTime QwtDateScaleEngine::toDateTime( double value ) const
{
    QDateTime dt = QwtDate::toDateTime( value, d_data->timeSpec );
    if ( !dt.isValid() )
    {
        const QDate date = ( value <= 0.0 ) 
            ? QwtDate::minDate() : QwtDate::maxDate();

        dt = QDateTime( date, QTime( 0, 0 ), d_data->timeSpec );
    }

    if ( d_data->timeSpec == Qt::OffsetFromUTC )
    {
        dt = dt.addSecs( d_data->utcOffset );
        dt.setUtcOffset( d_data->utcOffset );
    }

    return dt;
}