The global clock is an instance of the ClockObject class. It gets imported into the global namespace when you load the DirectStart/Showbase modules.
To get the time (in seconds) since the last frame was drawn:
dt = globalClock.getDt()
Another useful function is the frame time (in seconds, since the program started):
frameTime = globalClock.getFrameTime()
Note: this section is incomplete. It will be updated soon.