The global clock is an instance of the ClockObject class. There's a single instance of it already initialized that you can access statically.

To get the time (in seconds) since the last frame was drawn:

double dt = ClockObject::get_global_clock()->get_dt();

Another useful function is the frame time (in seconds, since the program started):

double frame_time = ClockObject::get_global_clock()->get_frame_time();

Incomplete Section

Note: this section is incomplete. It will be updated soon.