I'm not sure if its the same but on my d7200, the 'time' setting sets it up so when you press the shutter button and release it, the shutter stays open until the next shutter press. On the d7200, the top lcd shows '--' rather than 'bulb'. I'm not sure how it looks on the d3400, but I think that's the difference. Bulb you hold. Time, you press to start and press again to end.