Daylight Left is a data field for Garmin watches and bike computers that shows how much light is left in the day.
You might use it to figure out if you should cut an activity short to, for example, return to your car.
Daylight Left is available in the Garmin ConnectIQ Store
By default, Daylight Left will use sunset as its reference point. However, in practice, there's plenty of usable daylight left immediately after sunset.
To account for that, Daylight Left includes a setting, togglable from the ConnectIQ app on your phone, that allow you to use 'Twilight' and 'Dark' as reference points. These settings map to "civil" and "astronomical" sunsets which, interestingly, have precise definitions based on how far below the horizon the sun is.
Daylight Left is written in "Monkey C" for the ConnectIQ platform.
It uses an algorithm published by the National Almanac Office of the United States Naval Observatory sourced from here