Set the time you leave and how long your trip takes. The chart runs August to July — a whole academic year — and tells you exactly which stretch of it you will be arriving after the light has gone.
Reading it: the background is the sky — gold while the sun is up, a narrow lavender strip of twilight, then navy once it is properly dark. The indigo band is your trip and its solid top edge is the moment you get home — wherever that edge sits in the navy you are arriving after the light has gone, and that stretch turns orange. The vertical steps in the bands are the two clock changes; the faint dashed vertical is today.
How much the year swings is a function of latitude. College Station sits at 30.6° N, which is a mild version of it. Longest day minus shortest day, elsewhere:
Quito is the control case: sitting on the equator, its day length barely moves all year — two minutes between the longest day and the shortest. Fairbanks swings by eighteen hours. Everything you feel walking out into the dark in November is axial tilt, scaled by how far from the equator you happen to stand.
How this works. Nothing here is looked up or stored — the page solves the sun's position in your browser each time it loads, using the NOAA solar-position equations (after Meeus, Astronomical Algorithms); NOAA publishes the same method as its Solar Calculator. That is why it never expires and always starts from today. Results were checked against the U.S. Naval Observatory rise/set tables and agree to within about 20 seconds. Daylight-saving shifts come from your browser's own time-zone database, so clock changes stay right even if the rules change — they are the vertical breaks in both curves. "Dark" means the end of civil twilight, the sun 6° below the horizon, the conventional point for needing artificial light outdoors; sunset itself runs about 25 minutes earlier. A flat, unobstructed horizon is assumed — buildings, trees and terrain bring real darkness sooner. Coordinates are for College Station, Texas; anywhere else the whole picture shifts.