[KPhotoAlbum] Fwd: [Qt bugreports] Updates for QTBUG-75585: Massive performance regression in QML Date object

[Tobias Leupold]

Playing around a bit, we'll get UTC almost for free :-)

When reading the database, we can simply do QDateTime::toUTC() for all dates.
That will, in our case, assume the time to be in local timezone and return the
corresponding UTC representation.

Getting the timezone to put inside the database is a bit more tricky. We can
get the offset via:

    const int offsetFromUtc = QDateTime::currentDateTime().offsetFromUtc();
    const int hours = offsetFromUtc / 3600;
    const int minutes = (offsetFromUtc - hours * 3600) / 60;
    QString localTimezone = hours > 0 ? QStringLiteral("UTC+%1:%2")
                                      : QStringLiteral("UTC-%1:%2");
    localTimezone = localTimezone.arg(hours, 2, 10, QLatin1Char('0'));
    localTimezone = localTimezone.arg(minutes, 2, 10, QLatin1Char('0'));

But that also won't be unambiguous, as DST stuff is a political decision (man,
I really hope they finally abandon that time shifting in the EU, and they'll
stick to the normal "winter" time ... ;-) and may change for a defined period
of time.

So I think we should use a named timezone. We can get the local one via

    QTimeZone::systemTimeZone().id()

So I think the right approach would be (indside an update function):
 - Get QTimeZone::systemTimeZone().id() once
 - Get QDateTime::toUTC() for each image and put it in the database
 - Also put the local timezone in each image's data

Or we set a default timezone for the database and only put one into an image's
data if it's not the default one. This may cover most (if not all) images and
reduce overhead.

Of course, we'll need a possibility to adjust the image timezone for each
image. That won't be too hard, we can simply build a list of all available
ones via QTimeZone::availableTimeZoneIds() and let the user choose from that
for one image or a selection of images.

When showing the image, display the time as before if the image's timezone
matches the system's (or the database default one). If not, display it in the
image's timezone with a hint that the timezone shown deviates from the
standard.

What do you think?