1 files changed, 15 insertions, 14 deletions

diff --git a/src/corelib/tools/qtimezoneprivate.cpp b/src/corelib/tools/qtimezoneprivate.cpp
index 2b6c2ea6a5..8bf7336a42 100644
--- a/src/corelib/tools/qtimezoneprivate.cpp
+++ b/src/corelib/tools/qtimezoneprivate.cpp
@@ -260,6 +260,8 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
     const qint64 sixteenHoursInMSecs(16 * 3600 * 1000);
     Q_STATIC_ASSERT(-sixteenHoursInMSecs / 1000 < QTimeZone::MinUtcOffsetSecs
                   && sixteenHoursInMSecs / 1000 > QTimeZone::MaxUtcOffsetSecs);
+    const qint64 recent = forLocalMSecs - sixteenHoursInMSecs;
+    const qint64 imminent = forLocalMSecs + sixteenHoursInMSecs;
     /*
       Offsets are Local - UTC, positive to the east of Greenwich, negative to
       the west; DST offset always exceeds standard offset, when DST applies.
@@ -327,7 +329,7 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
 
         // Get a transition definitely before the local MSecs; usually all we need.
         // Only around the transition times might we need another.
-        Data tran = previousTransition(forLocalMSecs - sixteenHoursInMSecs);
+        Data tran = previousTransition(recent);
         Q_ASSERT(forLocalMSecs < 0 || // Pre-epoch TZ info may be unavailable
                  forLocalMSecs - tran.offsetFromUtc * 1000 >= tran.atMSecsSinceEpoch);
         Data nextTran = nextTransition(tran.atMSecsSinceEpoch);
@@ -343,8 +345,7 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
                && forLocalMSecs > nextTran.atMSecsSinceEpoch + nextTran.offsetFromUtc * 1000) {
             Data newTran = nextTransition(nextTran.atMSecsSinceEpoch);
             if (newTran.atMSecsSinceEpoch == invalidMSecs()
-                || newTran.atMSecsSinceEpoch + newTran.offsetFromUtc * 1000
-                > forLocalMSecs + sixteenHoursInMSecs) {
+                || newTran.atMSecsSinceEpoch + newTran.offsetFromUtc * 1000 > imminent) {
                 // Definitely not a relevant tansition: too far in the future.
                 break;
             }
@@ -357,10 +358,10 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
 
             /*
               So now tran is definitely before and nextTran is either after or only
-              slightly before.  The one with the larger offset is in DST; the other in
-              standard time.  Our hint tells us which of those to use (defaulting to
-              standard if no hint): try it first; if that fails, try the other; if both
-              fail life's tricky.
+              slightly before.  One is standard time; we interpret the other as DST
+              (although the transition might in fact by a change in standard offset).  Our
+              hint tells us which of those to use (defaulting to standard if no hint): try
+              it first; if that fails, try the other; if both fail, life's tricky.
             */
             Q_ASSERT(forLocalMSecs < 0
                      || forLocalMSecs - tran.offsetFromUtc * 1000 > tran.atMSecsSinceEpoch);
@@ -369,7 +370,9 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
             nextTran.atMSecsSinceEpoch = forLocalMSecs - nextTran.offsetFromUtc * 1000;
             tran.atMSecsSinceEpoch = forLocalMSecs - tran.offsetFromUtc * 1000;
 
-            const bool nextIsDst = tran.offsetFromUtc < nextTran.offsetFromUtc;
+            // If both or neither have zero DST, treat the one with lower offset as standard:
+            const bool nextIsDst = !nextTran.daylightTimeOffset == !tran.daylightTimeOffset
+                ? tran.offsetFromUtc < nextTran.offsetFromUtc : nextTran.daylightTimeOffset;
             // If that agrees with hint > 0, our first guess is to use nextTran; else tran.
             const bool nextFirst = nextIsDst == (hint > 0) && nextStart != invalidMSecs();
             for (int i = 0; i < 2; i++) {
@@ -399,7 +402,7 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
               0 < tran.atMSecsSinceEpoch - nextTran.atMSecsSinceEpoch
               = (nextTran.offsetFromUtc - tran.offsetFromUtc) * 1000
             */
-            int dstStep = nextTran.offsetFromUtc - tran.offsetFromUtc;
+            int dstStep = (nextTran.offsetFromUtc - tran.offsetFromUtc) * 1000;
             Q_ASSERT(dstStep > 0); // How else could we get here ?
             if (nextFirst) { // hint thought we needed nextTran, so use tran
                 tran.atMSecsSinceEpoch -= dstStep;
@@ -415,8 +418,8 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
     /* Bracket and refine to discover offset. */
     qint64 utcEpochMSecs;
 
-    int early = offsetFromUtc(forLocalMSecs - sixteenHoursInMSecs);
-    int late = offsetFromUtc(forLocalMSecs + sixteenHoursInMSecs);
+    int early = offsetFromUtc(recent);
+    int late = offsetFromUtc(imminent);
     if (Q_LIKELY(early == late)) { // > 99% of the time
         utcEpochMSecs = forLocalMSecs - early * 1000;
     } else {
@@ -437,9 +440,7 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
             utcEpochMSecs = forStd;
         } else {
             // Invalid forLocalMSecs: in spring-forward gap.
-            const int dstStep = daylightTimeOffset(early < late ?
-                                                   forLocalMSecs + sixteenHoursInMSecs :
-                                                   forLocalMSecs - sixteenHoursInMSecs);
+            const int dstStep = daylightTimeOffset(early < late ? imminent : recent);
             Q_ASSERT(dstStep); // There can't be a transition without it !
             utcEpochMSecs = (hint > 0) ? forStd - dstStep : forDst + dstStep;
         }