Right! In view of the above: At present you QTimer::start() and leave it to tick forever. Now, for a test, at some point before 71 minutes make your code stop that timer and completely restart it, or delete it and create a new one, or whatever. Do you no longer stop getting timeouts after 71 minutes? It's a workaround, if not a resolution :)
Good workaround discovered. Thanks to all who responded for your help on this!
Starting and stopping the QTimer did not help, nor did operating it in one-shot mode. However replacing it with a native periodic timer did prove to be a good workaround--it just emits a signal that is handled by the window. Experimentally, the gist of the QTimer-based code seems to work correctly in Linux, so I'm guessing the conjecture that the underlying clock source port has a limitation, e.g. counter word size, is most likely correct. I will take this up with the vendor anon.