WHAT TIME IS IT IN THE HORSEHEAD NEBULA?

Q. In a temporally ambiguous scene, what would you tag on to the slug line? As an example consider a submarine or starship. Probably DAY but more 'what does it really matter'. I've been using '2ND SHIFT' and the like but that's generally pretty useless so I was considering dropping it altogether.

DAY indicates mood and lighting as much as chronological time. So if you want a scene set on the bridge when just a skeleton crew is there (junior officers), and probably the lights slightly dimmed, set it in NIGHT, otherwise DAY. You can leave the time off, but it's a little jarring for the eye used to seeing DAY and NIGHT at the end of slugs.