date.qdoc

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// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page qml-qtqml-date.html
    \title The Date JavaScript Object
    \inqmlmodule QtQml
    \brief Provides date functions.
 
    The QML Date object extends the
    \l{ECMAScript Specification of Date}{JS Date object} with
    locale aware functions.
 
    Functions that accept a \c format argument take either Locale.LongFormat,
    Locale.ShortFormat, Locale.NarrowFormat enumeration values,
    or a string specifying the format.
 
    The form of the supported format strings is as described in the
    documentation of \l QDate::toString(), \l QTime::toString() and \l
    QDateTime::toString().
 
    If the date is invalid, an empty string is returned.
 
    \section1 Format Enumeration Values
 
    Use the enumeration values when you want a format that matches the locale
    preferences.
 
    \table
    \row \li Locale.LongFormat \li Longer format
    \row \li Locale.ShortFormat \li Shorter format
    \row \li Locale.NarrowFormat \li In this context same as Locale.ShortFormat
    \endtable
 
    The format that the enumerations represent will depend on your locale, but also
    the method that the enumeration is used for.
 
    For example, for the \c en_US locale, these format strings are used:
 
    \table
    \header \li Function \li Locale Enum \li Format String
    \row \li fromLocaleDateString, toLocaleDateString \li Locale.LongFormat \li \c{dddd, MMMM d, yyyy}
    \row \li fromLocaleDateString, toLocaleDateString \li Locale.ShortFormat \li \c{M/d/yy}
    \row \li fromLocaleTimeString, toLocaleTimeString \li Locale.LongFormat \li \c{h:mm:ss AP t}
    \row \li fromLocaleTimeString, toLocaleTimeString \li Locale.ShortFormat \li \c{h:mm AP}
    \row \li fromLocaleString, toLocaleString \li Locale.LongFormat \li \c{dddd, MMMM d, yyyy h:mm:ss AP t}
    \row \li fromLocaleString, toLocaleString \li Locale.ShortFormat \li \c{M/d/yy h:mm AP}
    \endtable
 
    \section1 Further Notes
 
    Using the locale-aware functions to perform date or time formatting can
    result in incorrectly formatted times, due to an inconsistency in specification
    between Qt and JS.  ECMA-262 specifies that historical dates should be intrepreted
    by projecting the current rules for daylight-saving onto past years, while Qt uses
    historical data (where available) to determine whether daylight-saving was in
    effect for a given date.  Therefore, constructing a Date value in JS and converting
    it to a string using the locale-aware functions can yield a result incorrect by one
    hour, if DST is currently in effect, while it was not for the time specified, or
    vice versa.
 
    There are different date formats with different understandings of negative years. Common
    human language does not have a year 0. The year after 1BC is 1AD. This understanding is
    reflected when printing or parsing dates in one of the formats not standardized by ECMAScript.
    That is: toString(), toLocaleString(), toUTCString() and friends. ECMAScript does standardize
    one format: ISO 8601. This is what you get when you call toISOString(). This format does include
    a year 0, which is 1BC in other formats. Thus you get different years when printing negative
    dates with toISOString() and toString().
 
    When setting the year using the Date constructor or set(UTC)FullYear(), the convention set by
    ISO 8601 is used and 0 is a valid year. This means negative years set with the constructor or
    set(UTC)FullYear() are zero-based and thus offset by one year from what is printed with
    toString() and friends. Parsing the output of any of the to*String() methods will yield the same
    date value you printed from. Date.parse() will recognize the different formats and their
    convention on the existence of year 0.
 
    Note that all this is different from what you get in other JavaScript implementations which
    usually treat year 0 as valid in all string representations. As the date formats are
    "implementation-dependent" in the ECMAScript standard, this is still valid, though.
 
    \sa {QtQml::Locale}{Locale}
 
    \section1 string Date::toLocaleString(locale, format)
 
    Converts the Date to a string containing the date and time
    suitable for the specified \a locale
    in the specified \a format.
 
    If \a format is not specified, \l {QtQml::Locale}{Locale.LongFormat} will
    be used.
 
    If \a locale is not specified, the default locale will be used.
 
    The following example shows the current date and time formatted
    for the German locale:
    \code
    import QtQuick 2.0
 
    Text {
        text: "The date is: " + new Date().toLocaleString(Qt.locale("de_DE"))
    }
    \endcode
 
    \section1 string Date::toLocaleDateString(locale, format)
 
    Converts the Date to a string containing the date suitable for the specified \a locale
    in the specified \a format.
 
    If \a format is not specified, \l {QtQml::Locale}{Locale.LongFormat} will
    be used.
 
    If \a locale is not specified, the default locale will be used.
 
    The following example shows the current date formatted
    for the German locale:
    \code
    import QtQuick 2.0
 
    Text {
        text: "The date is: " + new Date().toLocaleDateString(Qt.locale("de_DE"))
    }
    \endcode
 
    \section1 string Date::toLocaleTimeString(locale, format)
 
    Converts the Date to a string containing the time suitable for the specified \a locale
    in the specified \a format.
 
    If \a format is not specified, \l {QtQml::Locale}{Locale.LongFormat} will
    be used.
 
    If \a locale is not specified, the default locale will be used.
 
    The following example shows the current time formatted
    for the German locale:
    \code
    import QtQuick 2.0
 
    Text {
        text: "The date is: " + new Date().toLocaleTimeString(Qt.locale("de_DE"))
    }
    \endcode
 
    \section1 string Date::fromLocaleString(locale, dateTimeString, format)
 
    Converts the datetime string \a dateTimeString to a \l {The Date JavaScript Object}{Date}
    object using \a locale and \a format.
 
    If \a format is not specified, \l {QtQml::Locale}{Locale.LongFormat} will
    be used.
 
    If \a locale is not specified, the default locale will be used.
 
    The following example shows a datetime being parsed from a datetime string
    in a certain format using the default locale:
    \code
    import QtQml 2.0
 
    QtObject {
        property var locale: Qt.locale()
        property string dateTimeString: "Tue 2013-09-17 10:56:06"
 
        Component.onCompleted: {
            print(Date.fromLocaleString(locale, dateTimeString, "ddd yyyy-MM-dd hh:mm:ss"));
        }
    }
    \endcode
 
    \section1 string Date::fromLocaleDateString(locale, dateString, format)
 
    Converts the date string \a dateString to a \l {The Date JavaScript Object}{Date} object
    using \a locale and \a format.
 
    If \a format is not specified, \l {QtQml::Locale}{Locale.LongFormat} will
    be used.
 
    If \a locale is not specified, the default locale will be used.
 
    The following example shows the current date first being formatted as a date
    string using the default locale and format, then parsed back again in the
    same manner:
    \code
    import QtQml 2.0
 
    QtObject {
        property var locale: Qt.locale()
        property date currentDate: new Date()
        property string dateString
 
        Component.onCompleted: {
            dateString = currentDate.toLocaleDateString();
            print(Date.fromLocaleDateString(dateString));
        }
    }
    \endcode
 
    \section1 string Date::fromLocaleTimeString(locale, timeString, format)
 
    Converts the time string \a timeString to a \l {The Date JavaScript Object}{Date} object
    using \a locale and \a format.
 
    If \a format is not specified, \l {QtQml::Locale}{Locale.LongFormat} will
    be used.
 
    If \a locale is not specified, the default locale will be used.
 
    The following example shows the current time first being formatted as a time
    string using the default locale and a short format, then parsed back again
    in the same manner:
    \code
    import QtQml 2.2
 
    QtObject {
        property var locale: Qt.locale()
        property date currentTime: new Date()
        property string timeString
 
        Component.onCompleted: {
            timeString = currentTime.toLocaleTimeString(locale, Locale.ShortFormat);
            print(Date.fromLocaleTimeString(locale, timeString, Locale.ShortFormat));
        }
    }
    \endcode
 
    \section1 string Date::timeZoneUpdated()
 
    Informs the JS engine that the system's timezone has been changed, which is necessary
    for the correct manipulation of datetime data.
 
    JS stores Date objects in UTC time; all access to and from Date components in local
    time involves the application of the current offset from UTC.  If the current offset
    changes due to the timezone being updated, the JS engine needs to be informed so that
    it can recalculate the offset.
 
    This function should be called after the system's timezone has been updated.
 
    For example, an application that changes the timezone would call timeZoneUpdated() after
    setting the new time zone:
 
    \code
        property string selectedTimeZone
 
        onSelectedTimeZoneChanged: {
            MyFunctions.setSystemTimeZone(selectedTimeZone)
            Date.timeZoneUpdated()
        }
    \endcode
*/