A day/night indicator shows which half of the day corresponds to, typically, a 12-hour time display.
Day/night indicators are often paired with a 12-hour second timezone display, making it more clear which hour is shown. They are sometimes used on single-timezone watches as well, and often are paired with, or similar to, a moon phase indicator. Some day/night indicators are paired with a 24 hour display or are simply constructed as a disc replacing a 24-hour hand. Another popular day/night indicator is a two-sided hand with sun and moon ends which travels over a hemispherical 12-hour scale.
Many simple day/night indicators are a two-color disc. These often begin to "show" the next segment during the day or night: Many show "day" only from midnight to 6 AM and "night" only from noon to 6 PM, with a half-day or half-night display the rest of the time.