Sunset Standard Time is a 24-hour time standard that defines sunrise to be at 6:00am and sunset to be at 8:00pm. It is similar to apparent solar time, with adaptations to fit our modern forms of timekeeping.
Solar position comes from the NOAA solar-position algorithm evaluated in the browser; sunrise and sunset are found by bisecting the solar altitude curve rather than by a closed-form formula, which is what lets the polar cases fall out of the same code path.
Night has two lengths inside one SST day. Hours 00:00–06:00 are carved out of the night that is ending; hours 20:00–24:00 out of the night that is beginning. Those nights differ, so the first and last hours of an SST day are not the same size.
When the sun does not cross the horizon the anchor moves to the nearest usable threshold the sun's daily arc does contain — civil, nautical or astronomical twilight during polar night, and the mirrored +6°/+12°/+18° elevations during polar day — and finally clamps just inside the day's altitude range. The anchor in force is always named above.
+6°/+12°/+18°