Tells time until sunrise or sunset.
Wrist watches now have many extra functions; pulse, temperature, calculators, reminders, music, etc. The nature clock has functions useful to people outdoors.
The main feature is the sunrise/sunset mode. When in this mode the nature clock shows the time remaining until sunrise or sunset. To initialize this feature the user enters today's times for sunrise and sunset from a local paper. Today's sunrise and sunset times and date are all the information required to calculate ongoing sunrise and sunset times for the same geographical location. The feature would only have to be reset if the owner traveled a significant distance or changed the battery.
Additional outdoor functions such as tides, moon phase, moon rise, moon set, etc. might be added. The ability to set an alarm based on sunrise would be especially convenient to campers. Present watches have a number of features such as chronograph rings and deep waterproofing which are rarely used but convey an outdoorsy image. This would fall in that category.
A watch showing time until sunrise or sunset would sell to golfers, fishers, boaters, hunters, police, joggers, gardeners, campers, farmers, builders, anyone who works or plays outdoors.
This is a software invention and the main cost would be programming. There may be some difficulty in mountainous regions since the sun sets before it reaches the horizon; I doubt if published sunrise and sunset times reflect this. The calculation of sunrise and sunset times should be straightforward if the watch already maintains the current date on an internal calendar. Accuracy within a couple of minutes would be sufficient and would simplify the calculations. Daylight savings would be irrelevant since entering today's sunrise and sunset would include it. Many watches already contain countdown timer displays which could be set by the output from the sunrise or sunset calculation. A prototype desk clock could be built as a breadboard for debugging and demonstration.