Expand range of allowed UTC offsets to 16 hours

It turns out that Alaska and The Philippines had historical offsets
exceeding 15 hours, prior to day-transitions to bring their dates in
sync with their respective sides of the international date line.

Change-Id: I48fdf3aa6d8c0bacb368d08316733a10ee11a281
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qtimezone.cpp

@@ -177,7 +177,7 @@ Q_GLOBAL_STATIC(QTimeZoneSingleton, global_tz);
 
     A default UTC time zone backend is provided which is always available when
     feature \c timezone is enabled. This provides a set of generic Offset From
-    UTC time zones in the range UTC-14:00 to UTC+14:00. These time zones can be
+    UTC time zones in the range UTC-16:00 to UTC+16:00. These time zones can be
     created using either the standard ISO format names, such as "UTC+00:00", as
     listed by availableTimeZoneIds(), or using a name of similar form in
     combination with the number of offset seconds.
@@ -258,13 +258,27 @@ Q_GLOBAL_STATIC(QTimeZoneSingleton, global_tz);
 
   This enumeration provides constants bounding the range of plausible timezone
   offsets from UTC, measured in seconds.
-  Sane UTC offsets range from -14 to +14 hours.
-  No known zone has offset > 12 hrs West of Greenwich (Baker Island, USA).
-  No known zone has offset > 14 hrs East of Greenwich (Kiritimati, Christmas Island, Kiribati).
+
+  Sane modern zones' UTC offsets range from -14 to +12 hours.
+  No known modern zone has offset > 12 hrs West of Greenwich (Baker Island, USA).
+  No known modern zone has offset > 14 hrs East of Greenwich
+  (Kiritimati, Christmas Island, Kiribati).
   Note that there are zones whose offsets differ by more than a day.
 
-  \value MinUtcOffsetSecs -14 * 3600,
-  \value MaxUtcOffsetSecs +14 * 3600
+  Historically, before 1867, when Russia sold Alaska to America, Alaska used the
+  same date as Russia, so had offsets over 15 hours East of Greenwich.
+  Earlier still, until 1844, The Philippines (then controlled by Spain) used the
+  same date as Spain's American holdings, so had offsets close to 16 hours West
+  of Greenwich.
+  Each made its one-day transition to cross the international date line while
+  using local solar mean time, before adopting a unified time-zone, so the
+  offsets within their territories were variable.
+  As a result, The Philippines might have exceeded 16 hours as offset at the
+  extremities of its territory, but no modern zone's representative location was
+  at such an extremity.
+
+  \value MinUtcOffsetSecs -16 * 3600,
+  \value MaxUtcOffsetSecs +16 * 3600
 */
 
 #if QT_CONFIG(timezone)
@@ -468,7 +482,7 @@ QTimeZone::QTimeZone(const QByteArray &ianaId)
 /*!
     Creates a time zone instance with the given offset, \a offsetSeconds, from UTC.
 
-    The \a offsetSeconds from UTC must be in the range -14 hours to +14 hours
+    The \a offsetSeconds from UTC must be in the range -16 hours to +16 hours
     otherwise an invalid time zone will be returned.
 
     This constructor is only available when feature \c timezone is enabled. The
@@ -495,7 +509,7 @@ QTimeZone::QTimeZone(int offsetSeconds)
 
     The \a ianaId must not be one of the available system IDs returned by
     availableTimeZoneIds().  The \a offsetSeconds from UTC must be in the range
-    -14 hours to +14 hours.
+    -16 hours to +16 hours.
 
     If the custom time zone does not have a specific territory then set it to the
     default value of QLocale::AnyTerritory.
@@ -605,10 +619,10 @@ QTimeZone QTimeZone::asBackendZone() const
     Returns a time representation at a fixed \a offset, in seconds, ahead of
     UTC.
 
-    The \a offset from UTC must be in the range -14 hours to +14 hours otherwise an
-    invalid time zone will be returned. The returned QTimeZone is a lightweight
-    time representation, not a time zone (backed by system-supplied or standard
-    data).
+    The \a offset from UTC must be in the range -16 hours to +16 hours otherwise
+    an invalid time zone will be returned. The returned QTimeZone is a
+    lightweight time representation, not a time zone (backed by system-supplied
+    or standard data).
 
     If the offset is 0, the \l timeSpec() of the returned instance will be
     Qt::UTC. Otherwise, if \a offset is valid, timeSpec() is

src/corelib/time/qtimezone.h

@@ -82,12 +82,14 @@ class Q_CORE_EXPORT QTimeZone
     QTimeZone(ShortData sd) : d(sd) {}
 
 public:
-    // Sane UTC offsets range from -14 to +14 hours:
+    // Sane UTC offsets range from -16 to +16 hours:
     enum {
-        // No known zone > 12 hrs West of Greenwich (Baker Island, USA)
-        MinUtcOffsetSecs = -14 * 3600,
-        // No known zone > 14 hrs East of Greenwich (Kiritimati, Christmas Island, Kiribati)
-        MaxUtcOffsetSecs = +14 * 3600
+        // No known modern zone > 12 hrs West of Greenwich.
+        // Until 1844, Asia/Manila (in The Philippines) was at 15:56 West.
+        MinUtcOffsetSecs = -16 * 3600,
+        // No known modern zone > 14 hrs East of Greenwich.
+        // Until 1867, America/Metlakatla (in Alaska) was at 15:13:42 East.
+        MaxUtcOffsetSecs = +16 * 3600
     };
 
     enum Initialization { LocalTime, UTC };

src/corelib/time/qtimezoneprivate.cpp

@@ -182,28 +182,28 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
 
     /*
       We need a UTC time at which to ask for the offset, in order to be able to
-      add that offset to forLocalMSecs, to get the UTC time we
-      need. Fortunately, no time-zone offset is more than 14 hours; and DST
-      transitions happen (much) more than thirty-two hours apart.  So sampling
-      offset sixteen hours each side gives us information we can be sure
+      add that offset to forLocalMSecs, to get the UTC time we need.
+      Fortunately, all time-zone offsets have been less than 17 hours; and DST
+      transitions happen (much) more than thirty-four hours apart. So sampling
+      offset seventeen hours each side gives us information we can be sure
       brackets the correct time and at most one DST transition.
     */
-    std::integral_constant<qint64, 16 * 3600 * 1000> sixteenHoursInMSecs;
-    static_assert(-sixteenHoursInMSecs / 1000 < QTimeZone::MinUtcOffsetSecs
-                  && sixteenHoursInMSecs / 1000 > QTimeZone::MaxUtcOffsetSecs);
+    std::integral_constant<qint64, 17 * 3600 * 1000> seventeenHoursInMSecs;
+    static_assert(-seventeenHoursInMSecs / 1000 < QTimeZone::MinUtcOffsetSecs
+                  && seventeenHoursInMSecs / 1000 > QTimeZone::MaxUtcOffsetSecs);
     qint64 millis;
     // Clip the bracketing times to the bounds of the supported range. Exclude
     // minMSecs(), because at least one backend (Windows) uses it for a
     // start-of-time fake transition, that we want previousTransition() to find.
     const qint64 recent =
-        sub_overflow(forLocalMSecs, sixteenHoursInMSecs, &millis) || millis <= minMSecs()
+        sub_overflow(forLocalMSecs, seventeenHoursInMSecs, &millis) || millis <= minMSecs()
         ? minMSecs() + 1 : millis; // Necessarily <= forLocalMSecs + 2.
     // (Given that minMSecs() is std::numeric_limits<qint64>::min() + 1.)
     const qint64 imminent =
-        add_overflow(forLocalMSecs, sixteenHoursInMSecs, &millis)
+        add_overflow(forLocalMSecs, seventeenHoursInMSecs, &millis)
         ? maxMSecs() : millis; // Necessarily >= forLocalMSecs
     // At most one of those was clipped to its boundary value:
-    Q_ASSERT(recent < imminent && sixteenHoursInMSecs < imminent - recent + 2);
+    Q_ASSERT(recent < imminent && seventeenHoursInMSecs < imminent - recent + 2);
     /*
       Offsets are Local - UTC, positive to the east of Greenwich, negative to
       the west; DST offset always exceeds standard offset, when DST applies.

tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp

@@ -507,7 +507,7 @@ void tst_QTimeZone::asBackendZone()
 void tst_QTimeZone::systemZone()
 {
     const QTimeZone zone = QTimeZone::systemTimeZone();
-    QVERIFY(zone.isValid());
+    QVERIFY2(zone.isValid(), "Invalid system zone setting, tests are doomed.");
     QCOMPARE(zone.id(), QTimeZone::systemTimeZoneId());
     QCOMPARE(zone, QTimeZone(QTimeZone::systemTimeZoneId()));
     // Check it behaves the same as local-time:
@@ -1162,9 +1162,9 @@ void tst_QTimeZone::utcTest()
     QCOMPARE(tz.standardTimeOffset(now), 36000);
     QCOMPARE(tz.daylightTimeOffset(now), 0);
 
-    // Test invalid UTC offset, must be in range -14 to +14 hours
-    int min = -14*60*60;
-    int max = 14*60*60;
+    // Test validity range of UTC offsets:
+    int min = int(QTimeZone::MinUtcOffsetSecs);
+    int max = int(QTimeZone::MaxUtcOffsetSecs);
     QCOMPARE(QTimeZone(min - 1).isValid(), false);
     QCOMPARE(QTimeZone(min).isValid(), true);
     QCOMPARE(QTimeZone(min + 1).isValid(), true);