A turnaround time is the latest moment you can reverse toward the start and still be back before sunset, with a buffer you choose. Thirty minutes is a reasonable default. The calculator walks the route in pieces. It keeps the farthest piece where the time out plus the time back, including a share of your breaks, still fits in the daylight.
If the whole day fits, the card says so and the turnaround sits at the high point: you can continue, and that clock is the last time to change your mind. If it does not fit, the card names an earlier piece and turns on a headlamp warning. A headlamp is not permission to finish in the dark.
Sunset can be a time you type, or a local solar time from the date and a latitude and longitude. It is not a forecast. $5 unlocks the card, the printable view, and the CSV. The pack list is in Pack Planner.
Turnaround is a pace sketch from Naismith and Tobler, not a guide and not a weather report. Leave earlier for storms, a slow group, or a route you do not know. A headlamp does not make a late return safe.