When to take the roast out, when the flight lands, when the parking runs out.
The clock wraps and the “day” line says how far it travelled — next day, or “+2 days” for a 30-hour sous-vide. Subtracting works the same way backwards.
If your start time is the current time, that line anchors the result to the calendar — useful when “in 90 minutes” lands after midnight and you want to be sure which day it means.
Entirely in your browser. Dates you type are never sent anywhere — there is no server doing the math, no analytics watching you do it.