Use start of day when wrapping 24:00 to the next day

Previously we used 0:0 on the next day, which might fall in a
fall-back's gap.

[ChangeLog][QtCore][QDateTime] When fromString() reads 24:00 in ISO
format, it now uses the start of the next day, rather than 0:0 on the
next day. This only makes a difference if the next day's first hour is
skipped by a time-zone transition.

Change-Id: Ib81feca5dc09fa735321b6ab76d5d118d6db6fd2
Reviewed-by: Andreas Buhr <andreas.buhr@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

3 files changed, 24 insertions, 18 deletions

diff --git a/src/corelib/global/qnamespace.qdoc b/src/corelib/global/qnamespace.qdoc
index c81608ab4d..a877b17a4f 100644
--- a/src/corelib/global/qnamespace.qdoc
+++ b/src/corelib/global/qnamespace.qdoc
@@ -704,22 +704,23 @@
     \note For \c ISODate formats, each \c y, \c M and \c d represents a single
     digit of the year, month, and day used to specify the date. Each \c H, \c m,
     and \c s represents a single digit of the hour (up to 24), minute and second
-    used to specify the time. A \c{.zzz} stands for a fractional part suffix on
-    the preceding field, which may be separated from that field either by a
-    comma \c{','} or the dot \c{'.'} shown. Precision beyond milliseconds is
-    accepted but discarded, rounding to the nearest millisecond or, when
-    rounding fractional seconds up would change the second field, rounded
-    down. The presence of a literal \c T character is used to separate the date
-    and time when both are specified. For the \c TextDate and \c RFC2822Date
-    formats, \c{ddd} stands for the first three letters of the name of the day
-    of the week and \c{MMM} stands for the first three letters of the month
-    name. The names of days and months are always in English (C locale)
-    regardless of user preferences or system settings. The other format
-    characters have the same meaning as for the ISODate format. Parts of the
-    format enclosed in square brackets \c{[...]} are optional; the square
-    brackets do not form part of the format. The plus-or-minus character \c{'±'}
-    here stands for either sign character, \c{'-'} for minus or \c{'+'} for
-    plus.
+    used to specify the time. An hour of 24, with zero for all other time
+    fields, is understood as the start of the next day. A \c{.zzz} stands for a
+    fractional part suffix on the preceding field, which may be separated from
+    that field either by a comma \c{','} or the dot \c{'.'} shown. Precision
+    beyond milliseconds is accepted but discarded, rounding to the nearest
+    millisecond or, when rounding fractional seconds up would change the second
+    field, rounded down. The presence of a literal \c T character is used to
+    separate the date and time when both are specified. For the \c TextDate and
+    \c RFC2822Date formats, \c{ddd} stands for the first three letters of the
+    name of the day of the week and \c{MMM} stands for the first three letters
+    of the month name. The names of days and months are always in English (C
+    locale) regardless of user preferences or system settings. The other format
+    characters have the same meaning as for the ISODate format, except that 24
+    is not accepted as an hour. Parts of a format enclosed in square brackets
+    \c{[...]} are optional; the square brackets do not form part of the
+    format. The plus-or-minus character \c{'±'} here stands for either sign
+    character, \c{'-'} for minus or \c{'+'} for plus.
 
     \sa QDate::toString(), QTime::toString(), QDateTime::toString(),
         QDate::fromString(), QTime::fromString(), QDateTime::fromString()
diff --git a/src/corelib/time/qdatetime.cpp b/src/corelib/time/qdatetime.cpp
index 3ddad29080..ff4c57b1f8 100644
--- a/src/corelib/time/qdatetime.cpp
+++ b/src/corelib/time/qdatetime.cpp
@@ -4773,8 +4773,8 @@ QDateTime QDateTime::fromString(QStringView string, Qt::DateFormat format)
         QTime time = fromIsoTimeString(isoString, format, &isMidnight24);
         if (!time.isValid())
             return QDateTime();
-        if (isMidnight24)
-            date = date.addDays(1);
+        if (isMidnight24) // time is 0:0, but we want the start of next day:
+            return date.addDays(1).startOfDay(spec, offset);
         return QDateTime(date, time, spec, offset);
     }
     case Qt::TextDate: {
diff --git a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
index b414458466..18b7a9d74f 100644
--- a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
@@ -2297,6 +2297,11 @@ void tst_QDateTime::fromStringDateFormat_data()
     // 24:00:00 Should be next day according to ISO 8601 section 4.2.3.
     QTest::newRow("ISO 24:00") << QString::fromLatin1("2012-06-04T24:00:00")
         << Qt::ISODate << QDateTime(QDate(2012, 6, 5), QTime(0, 0), Qt::LocalTime);
+    QTest::newRow("ISO 24:00 in DST") // Only special if TZ=America/Sao_Paulo
+        << QString::fromLatin1("2008-10-18T24:00") << Qt::ISODate
+        << QDateTime(QDate(2008, 10, 19),
+                     QTime(QTimeZone::systemTimeZoneId() == "America/Sao_Paulo" ? 1 : 0, 0),
+                     Qt::LocalTime);
     QTest::newRow("ISO 24:00 end of month") << QString::fromLatin1("2012-06-30T24:00:00")
         << Qt::ISODate << QDateTime(QDate(2012, 7, 1), QTime(0, 0), Qt::LocalTime);
     QTest::newRow("ISO 24:00 end of year") << QString::fromLatin1("2012-12-31T24:00:00")