In the paragraph "Accuracy and Timer Resolution" of the QTimer documentation, it's written that "Timers will never time out earlier than the specified timeout value and they are not guaranteed to time out at the exact value specified. In many situations, they may time out late by a period of time that depends on the accuracy of the system timers."

Of course, it holds only to the Qt::PreciseTimer type. For both Qt::CoarseTimer and Qt::VeryCoarseTimer it may be +-(5%*interval) or +-1s, respectively.