Sorry for not replying sooner.

Short version (from a Gtk+ point of view): when you register with osso_context you can register for a DBus signal, sent when the display blanks/gets locked. Listen for that and stop all timers, saving off the current time and timer position. Stop updating the screen.

Similarly, there's a window property (or go off focus) which you can do similar with.

I don't know, OTTOMH, how to do these things in Qt; but those are the two main events you want to be listening for: not being on-top (and therefore visible) and the screen being locked/blanked (and therefore not being visible).