You should use QTimer in Qt applications. Usually you don't need to care about specific update time, as the goal is regular periodic check. So the most straightforward approarch is to create a timer for each feed and set the update interval of each timer (e.g. 10 minutes).
If you for some reason really want to make an update at specific time, you can use something like QDateTime::currentDateTime().msecsTo(targetTime)
to calculate timer interval, use QTimer::setSingleShot
to make the timer non-periodic and set another timer when the first one is expired.
It may be reasonable to do timer->setTimerType(Qt::VeryCoarseTimer)
because you don't need much accuracy and Qt can optimize performance and power consuming in some cases.
Note that you generally cannot use Python's means to set up timers because Qt has its own event loop and won't allow other libraries to run something in the middle of it.