Sluglines are the scene headings in your screenplay, usually broken into three components to specify where and when a scene is taking place.
INT. DANNY'S OFFICE. DAY
INT stands for INTERIOR. The action takes place in Danny's office, and it's daytime. If it was taking place outside Danny's office in the night time, it would read:
EXT. DANNY'S OFFICE. NIGHT
EXT stands for EXTERIOR.
You don't have to specify a time of day other than 'day' or 'night'. If you want to say it's dawn or some other specific time, keep that for the scene description. It's best to keep sluglines simple. Try to avoid sluglines that turn into elaborate sentences that only end up confusing and irritating everyone e.g.
INT. DANNY'S OFFICE. DAY - DUSK, FLASHBACK, MARIE'S POV, TEN YEARS AGO, WHEN SHE WAS JUST 7 AND DANNY WAS ONLY 10
Julie Gray on Formatting