Nintendo DS Clock
The clock is generally a very useful thing to use, as it is the known way in the real world and using any device to see the time. The options here simply allow you to do different things in regard to the time and date settings.
The first option just allows you to enter the right date so the game can keep track of the days and months of the year. There is nothing more to it than that.
The second option allows you to helpfully set your DS up to be an alarm clock that goes off at a specific time as entered here.
The final option is just to enter the current time so the DS clock shows the correct time.