Qt C++ Convert seconds to formatted string (hh:mm:ss)

Your timezone is included in it thats why. Try this:

QDateTime::fromTime_t(10).toUTC().toString("hh:mm:ss");

There is no QTime::fromTime_t; possibly you're using QDateTime::fromTime_t, which accounts for time zones and daylight savings.

Instead you can use QTime().addSecs(10).toString(...).