Shutter stays open for thirty minutes or until you press on the shutter a second time. With bulb the shutter only stays open whilst the shutter button is depressed or held open with the cable shutter lock held open, whereas with time the first shutter press opens the shutter and the second shutter press closes the shutter (or it times out after 30 minutes and closes the shutter).
Nikon do an app where you can download the manuals and view them on your phone, it's very useful to check some whilst out and about.