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QDateTime Class Reference
[QtCore module]

The QDateTime class provides date and time functions. More...

 #include <QDateTime>

Note: All functions in this class are reentrant.

• List of all members, including inherited members
• Qt 3 support members

Public Functions

Static Public Members

Related Non-Members

Detailed Description

The QDateTime class provides date and time functions.

A QDateTime object contains a calendar date and a clock time (a "datetime"). It is a combination of the QDate and QTime classes. It can read the current datetime from the system clock. It provides functions for comparing datetimes and for manipulating a datetime by adding a number of seconds, days, months, or years.

A QDateTime object is typically created either by giving a date and time explicitly in the constructor, or by using the static function currentDateTime() that returns a QDateTime object set to the system clock's time. The date and time can be changed with setDate() and setTime(). A datetime can also be set using the setTime_t() function that takes a POSIX-standard "number of seconds since 00:00:00 on January 1, 1970" value. The fromString() function returns a QDateTime, given a string and a date format used to interpret the date within the string.

The date() and time() functions provide access to the date and time parts of the datetime. The same information is provided in textual format by the toString() function.

QDateTime provides a full set of operators to compare two QDateTime objects where smaller means earlier and larger means later.

You can increment (or decrement) a datetime by a given number of seconds using addSecs(), or days using addDays(). Similarly you can use addMonths() and addYears(). The daysTo() function returns the number of days between two datetimes, and secsTo() returns the number of seconds between two datetimes.

QDateTime can store datetimes as local time or as UTC. QDateTime::currentDateTime() returns a QDateTime expressed as local time; use toUTC() to convert it to UTC. You can also use timeSpec() to find out if a QDateTime object stores a UTC time or a local time. Operations such as addSecs() and secsTo() are aware of daylight saving time (DST).

Note: QDateTime does not account for leap seconds.

Use of Gregorian and Julian Calendars

QDate uses the Gregorian calendar in all locales, beginning on the date 15 October 1582. For dates up to and including 4 October 1582, the Julian calendar is used. This means there is a 10-day gap in the internal calendar between the 4th and the 15th of October 1582. When you use QDateTime for dates in that epoch, the day after 4 October 1582 is 15 October 1582, and the dates in the gap are invalid.

The Julian to Gregorian changeover date used here is the date when the Gregorian calendar was first introduced, by Pope Gregory XIII. That change was not universally accepted and some localities only executed it at a later date (if at all). QDateTime doesn't take any of these historical facts into account. If an application must support a locale-specific dating system, it must do so on its own, remembering to convert the dates using the Julian day.

No Year 0

There is no year 0. Dates in that year are considered invalid. The year -1 is the year "1 before Christ" or "1 before current era." The day before 0001-01-01 is December 31st, 1 BCE.

Range of Valid Dates

The range of valid dates is from January 2nd, 4713 BCE, to sometime in the year 11 million CE. The Julian Day returned by QDate::toJulianDay() is a number in the contiguous range from 1 to overflow, even across QDateTime's "date holes". It is suitable for use in applications that must convert a QDateTime to a date in another calendar system, e.g., Hebrew, Islamic or Chinese.

The Gregorian calendar was introduced in different places around the world on different dates. QDateTime uses QDate to store the date, so it uses the Gregorian calendar for all locales, beginning on the date 15 October 1582. For dates up to and including 4 October 1582, QDateTime uses the Julian calendar. This means there is a 10-day gap in the QDateTime calendar between the 4th and the 15th of October 1582. When you use QDateTime for dates in that epoch, the day after 4 October 1582 is 15 October 1582, and the dates in the gap are invalid.

Use of System Timezone

QDateTime uses the system's time zone information to determine the offset of local time from UTC. If the system is not configured correctly or not up-to-date, QDateTime will give wrong results as well.

Daylight Savings Time (DST)

QDateTime takes into account the system's time zone information when dealing with DST. On modern Unix systems, this means it applies the correct historical DST data whenever possible. On Windows and Windows CE, where the system doesn't support historical DST data, historical accuracy is not maintained with respect to DST.

The range of valid dates taking DST into account is 1970-01-01 to the present, and rules are in place for handling DST correctly until 2037-12-31, but these could change. For dates falling outside that range, QDateTime makes a best guess using the rules for year 1970 or 2037, but we can't guarantee accuracy. This means QDateTime doesn't take into account changes in a locale's time zone before 1970, even if the system's time zone database supports that information.

See also QDate, QTime, and QDateTimeEdit.

Member Function Documentation

QDateTime::QDateTime ()

Constructs a null datetime (i.e. null date and null time). A null datetime is invalid, since the date is invalid.

See also isValid().

QDateTime::QDateTime ( const QDate & date )

Constructs a datetime with the given date, a valid time(00:00:00.000), and sets the timeSpec() to Qt::LocalTime.

QDateTime::QDateTime ( const QDate & date, const QTime & time, Qt::TimeSpec spec = Qt::LocalTime )

Constructs a datetime with the given date and time, using the time specification defined by spec.

If date is valid and time is not, the time will be set to midnight.

QDateTime::QDateTime ( const QDateTime & other )

Constructs a copy of the other datetime.

QDateTime::~QDateTime ()

Destroys the datetime.

QDateTime QDateTime::addDays ( int ndays ) const

Returns a QDateTime object containing a datetime ndays days later than the datetime of this object (or earlier if ndays is negative).

See also daysTo(), addMonths(), addYears(), and addSecs().

QDateTime QDateTime::addMSecs ( qint64 msecs ) const

Returns a QDateTime object containing a datetime msecs miliseconds later than the datetime of this object (or earlier if msecs is negative).

See also addSecs(), secsTo(), addDays(), addMonths(), and addYears().

QDateTime QDateTime::addMonths ( int nmonths ) const

Returns a QDateTime object containing a datetime nmonths months later than the datetime of this object (or earlier if nmonths is negative).

See also daysTo(), addDays(), addYears(), and addSecs().

QDateTime QDateTime::addSecs ( int s ) const

Returns a QDateTime object containing a datetime s seconds later than the datetime of this object (or earlier if s is negative).

See also addMSecs(), secsTo(), addDays(), addMonths(), and addYears().

QDateTime QDateTime::addYears ( int nyears ) const

Returns a QDateTime object containing a datetime nyears years later than the datetime of this object (or earlier if nyears is negative).

See also daysTo(), addDays(), addMonths(), and addSecs().

QDateTime QDateTime::currentDateTime ()   [static]

Returns the current datetime, as reported by the system clock, in the local time zone.

See also QDate::currentDate(), QTime::currentTime(), and toTimeSpec().

QDate QDateTime::date () const

Returns the date part of the datetime.

See also setDate(), time(), and timeSpec().

int QDateTime::daysTo ( const QDateTime & other ) const

Returns the number of days from this datetime to the other datetime. If the other datetime is earlier than this datetime, the value returned is negative.

See also addDays() and secsTo().

QDateTime QDateTime::fromString ( const QString & string, Qt::DateFormat format = Qt::TextDate )   [static]

Returns the QDateTime represented by the string, using the format given, or an invalid datetime if this is not possible.

Note for Qt::TextDate: It is recommended that you use the English short month names (e.g. "Jan"). Although localized month names can also be used, they depend on the user's locale settings.

QDateTime QDateTime::fromString ( const QString & string, const QString & format )   [static]

Returns the QDateTime represented by the string, using the format given, or an invalid datetime if the string cannot be parsed.

These expressions may be used for the date part of the format string:

Note: Unlike the other version of this function, day and month names must be given in the user's local language. It is only possible to use the English names if the user's language is English.

These expressions may be used for the time part of the format string:

All other input characters will be treated as text. Any sequence of characters that are enclosed in singlequotes will also be treated as text and not be used as an expression.

 QTime time1 = QTime::fromString("131", "HHh");
 // time1 is 13:00:00
 QTime time1 = QTime::fromString("1apA", "1amAM");
 // time1 is 01:00:00

 QDateTime dateTime2 = QDateTime::fromString("M1d1y9800:01:02",
                                             "'M'M'd'd'y'yyhh:mm:ss");
 // dateTime is 1 January 1998 00:01:02

If the format is not satisfied an invalid QDateTime is returned. The expressions that don't have leading zeroes (d, M, h, m, s, z) will be greedy. This means that they will use two digits even if this will put them outside the range and/or leave too few digits for other sections.

 QDateTime dateTime = QDateTime::fromString("130", "Mm"); // invalid

This could have meant 1 January 00:30.00 but the M will grab two digits.

For any field that is not represented in the format the following defaults are used:

For example:

 QDateTime dateTime = QDateTime::fromString("1.30.1", "M.d.s");
 // dateTime is January 30 in 1900 at 00:00:01.

QDateTime::toString() QTime::toString()

See also QDate::fromString(), QTime::fromString(), and QDate::toString().

QDateTime QDateTime::fromTime_t ( uint seconds )   [static]

Returns a datetime whose date and time are the number of seconds that have passed since 1970-01-01T00:00:00, Coordinated Universal Time (Qt::UTC). On systems that do not support time zones, the time will be set as if local time were Qt::UTC.

This function was introduced in Qt 4.2.

See also toTime_t() and setTime_t().

bool QDateTime::isNull () const

Returns true if both the date and the time are null; otherwise returns false. A null datetime is invalid.

See also QDate::isNull(), QTime::isNull(), and isValid().

bool QDateTime::isValid () const

Returns true if both the date and the time are valid; otherwise returns false.

See also QDate::isValid() and QTime::isValid().

int QDateTime::secsTo ( const QDateTime & other ) const

Returns the number of seconds from this datetime to the other datetime. If the other datetime is earlier than this datetime, the value returned is negative.

Before performing the comparison, the two datetimes are converted to Qt::UTC to ensure that the result is correct if one of the two datetimes has daylight saving time (DST) and the other doesn't.

Example:

 QDateTime now = QDateTime::currentDateTime();
 QDateTime xmas(QDate(now.date().year(), 12, 25), QTime(0, 0));
 qDebug("There are %d seconds to Christmas", now.secsTo(xmas));

See also addSecs(), daysTo(), and QTime::secsTo().

void QDateTime::setDate ( const QDate & date )

Sets the date part of this datetime to date. If no time is set, it is set to midnight.

See also date(), setTime(), and setTimeSpec().

void QDateTime::setTime ( const QTime & time )

Sets the time part of this datetime to time.

See also time(), setDate(), and setTimeSpec().

void QDateTime::setTimeSpec ( Qt::TimeSpec spec )

Sets the time specification used in this datetime to spec.

See also timeSpec(), setDate(), setTime(), and Qt::TimeSpec.

void QDateTime::setTime_t ( uint seconds )

Sets the date and time given the number of seconds that have passed since 1970-01-01T00:00:00, Coordinated Universal Time (Qt::UTC). On systems that do not support time zones this function will behave as if local time were Qt::UTC.

See also toTime_t().

QTime QDateTime::time () const

Returns the time part of the datetime.

See also setTime(), date(), and timeSpec().

Qt::TimeSpec QDateTime::timeSpec () const

Returns the time specification of the datetime.

See also setTimeSpec(), date(), time(), and Qt::TimeSpec.

QDateTime QDateTime::toLocalTime () const

Returns a datetime containing the date and time information in this datetime, but specified using the Qt::LocalTime definition.

See also toTimeSpec().

QString QDateTime::toString ( const QString & format ) const

Returns the datetime as a string. The format parameter determines the format of the result string.

These expressions may be used for the date:

These expressions may be used for the time:

All other input characters will be ignored. Any sequence of characters that are enclosed in singlequotes will be treated as text and not be used as an expression. Two consecutive singlequotes ("''") are replaced by a singlequote in the output.

Example format strings (assumed that the QDateTime is 21 May 2001 14:13:09):

If the datetime is invalid, an empty string will be returned.

See also QDate::toString() and QTime::toString().

QString QDateTime::toString ( Qt::DateFormat format = Qt::TextDate ) const

This is an overloaded function.

Returns the datetime as a string in the format given.

If the format is Qt::TextDate, the string is formatted in the default way. QDate::shortDayName(), QDate::shortMonthName(), and QTime::toString() are used to generate the string, so the day and month names will be localized names. An example of this formatting is "Wed May 20 03:40:13 1998".

If the format is Qt::ISODate, the string format corresponds to the ISO 8601 extended specification for representations of dates and times, taking the form YYYY-MM-DDTHH:MM:SS.

If the format is Qt::SystemLocaleShortDate or Qt::SystemLocaleLongDate, the string format depends on the locale settings of the system. Identical to calling QLocale::system().toString(datetime, QLocale::ShortFormat) or QLocale::system().toString(datetime, QLocale::LongFormat).

If the format is Qt::DefaultLocaleShortDate or Qt::DefaultLocaleLongDate, the string format depends on the default application locale. This is the locale set with QLocale::setDefault(), or the system locale if no default locale has been set. Identical to calling QLocale().toString(datetime, QLocale::ShortFormat) or QLocale().toString(datetime, QLocale::LongFormat).

If the datetime is invalid, an empty string will be returned.

Warning: The Qt::ISODate format is only valid for years in the range 0 to 9999. This restriction may apply to locale-aware formats as well, depending on the locale settings.

See also QDate::toString(), QTime::toString(), and Qt::DateFormat.

QDateTime QDateTime::toTimeSpec ( Qt::TimeSpec specification ) const

Returns a copy of this datetime configured to use the given time specification.

See also timeSpec(), toUTC(), and toLocalTime().

uint QDateTime::toTime_t () const

Returns the datetime as the number of seconds that have passed since 1970-01-01T00:00:00, Coordinated Universal Time (Qt::UTC).

On systems that do not support time zones, this function will behave as if local time were Qt::UTC.

See also setTime_t().

QDateTime QDateTime::toUTC () const

Returns a datetime containing the date and time information in this datetime, but specified using the Qt::UTC definition.

See also toTimeSpec().

bool QDateTime::operator!= ( const QDateTime & other ) const

Returns true if this datetime is different from the other datetime; otherwise returns false.

Two datetimes are different if either the date, the time, or the time zone components are different.

See also operator==().

bool QDateTime::operator< ( const QDateTime & other ) const

Returns true if this datetime is earlier than the other datetime; otherwise returns false.

bool QDateTime::operator<= ( const QDateTime & other ) const

Returns true if this datetime is earlier than or equal to the other datetime; otherwise returns false.

QDateTime & QDateTime::operator= ( const QDateTime & other )

Makes a copy of the other datetime and returns a reference to the copy.

bool QDateTime::operator== ( const QDateTime & other ) const

Returns true if this datetime is equal to the other datetime; otherwise returns false.

See also operator!=().

bool QDateTime::operator> ( const QDateTime & other ) const

Returns true if this datetime is later than the other datetime; otherwise returns false.

bool QDateTime::operator>= ( const QDateTime & other ) const

Returns true if this datetime is later than or equal to the other datetime; otherwise returns false.

Related Non-Members

QDataStream & operator<< ( QDataStream & out, const QDateTime & dateTime )

Writes dateTime to the out stream.

See also Format of the QDataStream operators.

QDataStream & operator>> ( QDataStream & in, QDateTime & dateTime )

Reads a datetime from the stream in into dateTime.

See also Format of the QDataStream operators.