QDateTime: add support for std::chrono::duration arithmetic

QDateTime represents a specific point in time, so arithmetic
with durations makes perfect sense.

Moreover, we can finally equip QDateTime with a subtraction
operator, to calculate the duration between two QDateTime
objects.

[ChangeLog][QtCore][QDateTime] QDateTime now supports arithmetic
between QDateTime objects and std::chrono::duration objects.
A duration can be added to or subtracted from a QDateTime, yielding
another QDateTime; and two QDateTime objects can be subtracted
from each other, yielding the duration between them.

Change-Id: I656419f3bb9418c49f0e2fd0800c3dbaaf6aff32
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qdatetime.cpp

@@ -4622,6 +4622,75 @@ qint64 QDateTime::msecsTo(const QDateTime &other) const
     return other.toMSecsSinceEpoch() - toMSecsSinceEpoch();
 }
 
+/*!
+    \fn std::chrono::milliseconds QDateTime::operator-(const QDateTime &lhs, const QDateTime &rhs)
+    \since 6.4
+
+    Returns the number of milliseconds between \a lhs and \a rhs.
+    If \a lhs is earlier than \a rhs, the result will be negative.
+
+    Returns 0 if either datetime is invalid.
+
+    \sa msecsTo()
+*/
+
+/*!
+    \fn QDateTime QDateTime::operator+(const QDateTime &dateTime, std::chrono::milliseconds duration)
+    \fn QDateTime QDateTime::operator+(std::chrono::milliseconds duration, const QDateTime &dateTime)
+
+    \since 6.4
+
+    Returns a QDateTime object containing a datetime \a duration milliseconds
+    later than \a dateTime (or earlier if \a duration is negative).
+
+    If \a dateTime is invalid, an invalid datetime will be returned.
+
+    \sa addMSecs()
+*/
+
+/*!
+    \fn QDateTime &QDateTime::operator+=(std::chrono::milliseconds duration)
+    \since 6.4
+
+    Modifies this datetime object by adding the given \a duration.
+    The updated object will be later if \a duration is positive,
+    or earlier if it is negative.
+
+    If this datetime is invalid, this function has no effect.
+
+    Returns a reference to this datetime object.
+
+    \sa addMSecs()
+*/
+
+/*!
+    \fn QDateTime QDateTime::operator-(const QDateTime &dateTime, std::chrono::milliseconds duration)
+
+    \since 6.4
+
+    Returns a QDateTime object containing a datetime \a duration milliseconds
+    earlier than \a dateTime (or later if \a duration is negative).
+
+    If \a dateTime is invalid, an invalid datetime will be returned.
+
+    \sa addMSecs()
+*/
+
+/*!
+    \fn QDateTime &QDateTime::operator-=(std::chrono::milliseconds duration)
+    \since 6.4
+
+    Modifies this datetime object by subtracting the given \a duration.
+    The updated object will be earlier if \a duration is positive,
+    or later if it is negative.
+
+    If this datetime is invalid, this function has no effect.
+
+    Returns a reference to this datetime object.
+
+    \sa addMSecs
+*/
+
 /*!
     Returns a copy of this datetime converted to the given time
     \a spec.

src/corelib/time/qdatetime.h

@@ -47,6 +47,7 @@
 #include <QtCore/qcalendar.h>
 
 #include <limits>
+#include <chrono>
 
 #if defined(Q_OS_DARWIN) || defined(Q_QDOC)
 Q_FORWARD_DECLARE_CF_TYPE(CFDate);
@@ -385,6 +386,38 @@ public:
     NSDate *toNSDate() const Q_DECL_NS_RETURNS_AUTORELEASED;
 #endif
 
+    friend std::chrono::milliseconds operator-(const QDateTime &lhs, const QDateTime &rhs)
+    {
+        return std::chrono::milliseconds(rhs.msecsTo(lhs));
+    }
+
+    friend QDateTime operator+(const QDateTime &dateTime, std::chrono::milliseconds duration)
+    {
+        return dateTime.addMSecs(duration.count());
+    }
+
+    friend QDateTime operator+(std::chrono::milliseconds duration, const QDateTime &dateTime)
+    {
+        return dateTime + duration;
+    }
+
+    QDateTime &operator+=(std::chrono::milliseconds duration)
+    {
+        *this = addMSecs(duration.count());
+        return *this;
+    }
+
+    friend QDateTime operator-(const QDateTime &dateTime, std::chrono::milliseconds duration)
+    {
+        return dateTime.addMSecs(-duration.count());
+    }
+
+    QDateTime &operator-=(std::chrono::milliseconds duration)
+    {
+        *this = addMSecs(-duration.count());
+        return *this;
+    }
+
     // (1<<63) ms is 292277024.6 (average Gregorian) years, counted from the start of 1970, so
     // Last is floor(1970 + 292277024.6); no year 0, so First is floor(1970 - 1 - 292277024.6)
     enum class YearRange : qint32 { First = -292275056,  Last = +292278994 };

tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp

@@ -1483,14 +1483,29 @@ void tst_QDateTime::addSecs()
     QFETCH(const qint64, nsecs);
     QFETCH(const QDateTime, result);
     QDateTime test = dt.addSecs(nsecs);
+    QDateTime test2 = dt + std::chrono::seconds(nsecs);
+    QDateTime test3 = dt;
+    test3 += std::chrono::seconds(nsecs);
     if (!result.isValid()) {
         QVERIFY(!test.isValid());
+        QVERIFY(!test2.isValid());
+        QVERIFY(!test3.isValid());
     } else {
         QCOMPARE(test, result);
+        QCOMPARE(test2, result);
+        QCOMPARE(test3, result);
         QCOMPARE(test.timeSpec(), dt.timeSpec());
-        if (test.timeSpec() == Qt::OffsetFromUTC)
+        QCOMPARE(test2.timeSpec(), dt.timeSpec());
+        QCOMPARE(test3.timeSpec(), dt.timeSpec());
+        if (test.timeSpec() == Qt::OffsetFromUTC) {
             QCOMPARE(test.offsetFromUtc(), dt.offsetFromUtc());
+            QCOMPARE(test2.offsetFromUtc(), dt.offsetFromUtc());
+            QCOMPARE(test3.offsetFromUtc(), dt.offsetFromUtc());
+        }
         QCOMPARE(result.addSecs(-nsecs), dt);
+        QCOMPARE(result - std::chrono::seconds(nsecs), dt);
+        test3 -= std::chrono::seconds(nsecs);
+        QCOMPARE(test3, dt);
     }
 }
 
@@ -1744,7 +1759,9 @@ void tst_QDateTime::msecsTo()
 
     if (result.isValid()) {
         QCOMPARE(dt.msecsTo(result), qint64(nsecs) * 1000);
+        QCOMPARE(result - dt, std::chrono::milliseconds(nsecs * 1000));
         QCOMPARE(result.msecsTo(dt), -qint64(nsecs) * 1000);
+        QCOMPARE(dt - result, -std::chrono::milliseconds(nsecs * 1000));
         QVERIFY((dt == result) == (0 == (qint64(nsecs) * 1000)));
         QVERIFY((dt != result) == (0 != (qint64(nsecs) * 1000)));
         QVERIFY((dt < result) == (0 < (qint64(nsecs) * 1000)));
@@ -1753,7 +1770,9 @@ void tst_QDateTime::msecsTo()
         QVERIFY((dt >= result) == (0 >= (qint64(nsecs) * 1000)));
     } else {
         QVERIFY(dt.msecsTo(result) == 0);
+        QCOMPARE(result - dt, std::chrono::milliseconds(0));
         QVERIFY(result.msecsTo(dt) == 0);
+        QCOMPARE(dt - result, std::chrono::milliseconds(0));
     }
 }