Qt QDateTime from string with timezone and daylight saving - c++

Question

i am inserting time from string
QDateTime time =QDateTime::fromString("Wed Mar 26 22:37:40 2019 GMT-08");
qDebug()<<time.toLocalTime().toString();
qDebug()<<time.toUTC().toString();
qDebug()<<time.isDaylightTime();
output i am getting as
"Tue Mar 26 23:37:40 2019"
"Wed Mar 27 06:37:40 2019 GMT"
false
it should have given
"Tue Mar 26 23:37:40 2019"
"Wed Mar 27 05:37:40 2019 GMT"
true
how can i pass daylight saving with the timestamp string?

Answer 1

If you take a look into the official docs, it says:
If the Qt::TimeSpec is not Qt::LocalTime or Qt::TimeZone then will always return false.
So first of all, check that the QDateTime::timeSpec is returning what you expect.
If you know the format in advance, try to specify the format of the string that you want to parse by using the equivalent function QDateTime::fromString.
Combining both things you can do something like this:
const char* s = "2009-11-05T03:54:00";
QDateTime d = QDateTime::fromString(s, Qt::ISODate).toLocalTime();
d.setTimeSpec(Qt::LocalTime); // Just to ensure that the time spec are the proper one, i think is not needed
qDebug() << d.isDaylightTime();

Answer 2

First, UTC time "Wed Mar 27 06:37:40 2019 GMT" is definitely right when calculated from "Wed Mar 26 22:37:40 2019 GMT-08". How do you think it could be 5:37?
Explanation why GMT or UTC doesn't include DST:
Neither UTC nor GMT ever change for Daylight Saving Time (DST).
However, some of the countries that use GMT switch to different time
zones during their DST period. For example, AKDT (Alaska Daylight
Time) is one of GMT-8 time zones during their DST (summer daylight
saving time) between 10 March and 3 November in 2019. During the
winter, AKST (Alaska Standard Time) which is GMT-9 is in use.
Second, as was already pointed in the other answer time QDateTime::isDaylightTime always returns false if the Qt::TimeSpec is not Qt::LocalTime or Qt::TimeZone.
When you use QDateTime::fromString with time zone information as in your code example timespec is correctly set to Qt::OffsetFromUTC. You can instantiate another QDateTime object to the same time but having the TimeSpec as Qt::LocalTime or Qt::TimeZone. You can e.g. convert to local time with QDateTime::toLocalTime or to either Qt::LocalTime or Qt::TimeZone with QDateTime::fromSecsSinceEpoch which accepts offset seconds for time zone.
See example code below. I'm located in Finland where daylight savings time starts in March 31 so you can see difference of local time when standard time is in use and when daylight time is in use:
QDateTime time = QDateTime::fromString("Wed Mar 26 22:37:40 2019 GMT-08");
qDebug()<<"\nLocal time EET:";
QDateTime localTime = time.toLocalTime();
// This works too, here to local time:
//QDateTime localTime = QDateTime::fromSecsSinceEpoch(time.toSecsSinceEpoch());
qDebug()<<localTime.timeSpec();
qDebug()<<localTime.timeZone();
qDebug()<<localTime.timeZoneAbbreviation();
qDebug()<<localTime.toLocalTime().toString();
qDebug()<<localTime.toUTC().toString();
qDebug()<<localTime.isDaylightTime();
time = QDateTime::fromString("Wed Apr 26 22:37:40 2019 GMT-08");
qDebug()<<"\nLocal time EEST:";
localTime = time.toLocalTime();
qDebug()<<localTime.timeSpec();
qDebug()<<localTime.timeZone();
qDebug()<<localTime.timeZoneAbbreviation();
qDebug()<<localTime.toLocalTime().toString();
qDebug()<<localTime.toUTC().toString();
qDebug()<<localTime.isDaylightTime();
Output:
Local time EET:
Qt::LocalTime
QTimeZone("Europe/Helsinki")
"EET"
"Wed Mar 27 08:37:40 2019"
"Wed Mar 27 06:37:40 2019 GMT"
false
Local time EEST:
Qt::LocalTime
QTimeZone("Europe/Helsinki")
"EEST"
"Sat Apr 27 09:37:40 2019"
"Sat Apr 27 06:37:40 2019 GMT"
true