diff options
| -rw-r--r-- | src/corelib/time/qtimezoneprivate_win.cpp | 22 |
1 files changed, 21 insertions, 1 deletions
diff --git a/src/corelib/time/qtimezoneprivate_win.cpp b/src/corelib/time/qtimezoneprivate_win.cpp index ff8cd5548e..06524f1189 100644 --- a/src/corelib/time/qtimezoneprivate_win.cpp +++ b/src/corelib/time/qtimezoneprivate_win.cpp @@ -73,6 +73,22 @@ static const wchar_t currTzRegPath[] = LR"(SYSTEM\CurrentControlSet\Control\Time constexpr qint64 MSECS_PER_DAY = 86400000LL; constexpr qint64 JULIAN_DAY_FOR_EPOCH = 2440588LL; // result of julianDayFromDate(1970, 1, 1) +/* Ignore any claims of DST before 1900. + + Daylight-Saving time adjustments were first proposed in 1895 (George Vernon + Hudson in New Zealand) and 1905 (William Willett in the UK) and first adopted + in 1908 (one town in Ontario, Canada) and 1916 (Germany). Since MS's data + tends to pretend the rules in force in 1970ish (or later) had always been in + effect, which presents difficulties for the code that selects correct data + (for a time close to the earliest we can represent), always ignore any claims + a first rule may make of DST before 1900. + + See: + * https://www.timeanddate.com/time/dst/history.html + * https://en.wikipedia.org/wiki/Daylight_saving_time#History +*/ +constexpr int FIRST_DST_YEAR = 1900; + // Copied from MSDN, see above for link typedef struct _REG_TZI_FORMAT { @@ -606,7 +622,11 @@ QTimeZonePrivate::Data QWinTimeZonePrivate::data(qint64 forMSecsSinceEpoch) cons : yearEndOffset(rule, prior); const TransitionTimePair pair(rule, year, newYearOffset); bool isDst = false; - if (pair.std != invalidMSecs() && pair.std <= forMSecsSinceEpoch) { + if (!ruleIndex && year < FIRST_DST_YEAR) { + // We're before the invention of DST and have no earlier + // rule that might give better data on this year, so just + // extrapolate standard time (modulo fakery) backwards. + } else if (pair.std != invalidMSecs() && pair.std <= forMSecsSinceEpoch) { isDst = pair.std < pair.dst && pair.dst <= forMSecsSinceEpoch; } else if (pair.dst != invalidMSecs() && pair.dst <= forMSecsSinceEpoch) { isDst = true; |