How TheDarkestDay Works Out Sunrise, Sunset and the Dark Days
The sun, calculated for each town
For each of 3,206 US Census places, the sun's position is calculated every day from July 2026 to June 2027 using the NOAA Solar Calculator equations (Meeus, Astronomical Algorithms). Sunrise and sunset are when the top of the sun crosses a flat horizon, allowing for the air bending its light, which is the same convention almanacs use: a sun altitude of -0.833°. Twilight ends at -6°, civil twilight, roughly when you need street lights to see. Run for the shortest day, that gives Fairbanks, AK 3h 42m of daylight, Honolulu, HI 10h 50m, and New York, NY 9h 16m in between.
Clock time, not sun time
Every time is converted to the place's own IANA time zone, daylight saving included. That matters more than people expect: two towns at the same latitude can see sunset half an hour apart because one sits at the eastern edge of its time zone and one at the western edge. The earliest and latest sunsets guide shows how big that gets.
The two assumptions, stated
"Waking in the dark" assumes a 7:00 am alarm, the later end of the typical workday wake-up in the BLS American Time Use Survey, so the count is cautious rather than inflated. "Coming home in the dark" assumes you are outside at 5:30 pm, a 9-to-5 job plus the Census average commute of about 27 minutes. Only weekdays are counted, because the alarm question is a workday question. A morning counts as properly dark when the sun is still at least 20 minutes from rising.
What "strong" and "some" mean
A sunrise alarm or lamp case is called strong at 60 or more qualifying weekdays in a season, about three months of workdays, and "some" at 20 or more, about one month. These cut-offs are judgement calls chosen to mean something in calendar terms; they are not a clinical threshold. The alarm and lamp guide shows how the country splits under them.