The power reserve indicator tells you the time the watch will continue to function. It can either be seen on hand winding or automatic watches.
The perpetual calendar complication displays the day, date and month, but it requires less adjustment than an annual calendar. This complication knows the number of days in each month but it also knows the year so that it will automatically adjust for leap year. The next time a perpetual calendar will have to be adjusted is in the year 2100.
The equation of time is the difference between true solar time and mean time. True solar time varies from day to day because of the Earth's elliptical orbit. Mean time, given by watches, ignores these variations and for every day of the year mathematically divides time into equal hours.
The night and day indicator is an aesthetic complication which displays on the dial the movement of the sun and the moon. This complication must not be confused with the moonphase.
The tourbillon system was made to compensate the Earth’s gravitational force by mounting the escapement in a revolving cage. It gives the watch a better accuracy. This complication is known as one of the most difficult to make.