Calendar watches display the date as well as the time. In general, they indicate the date, the day of the week and the month. The simple date is a complication which takes no account of months with less than 31 days. It requires a manual correction for months of 30 days and the month of February.
The perpetual calendar is a complication which takes account of all the months of the year, i.e. months with 28, 30 and 31 days, but also the month of February with 29 days or the leap year which occurs once every four years. To achieve this feat, the movement must have a mechanical memory of 1,461 days.
The century calendar is a device which changes the date at the end of each month while also taking account of the 400 year cycles of the Gregorian calendar (elimination of the 29th day in the month of February in the final year of the century when the figure representing the thousands, excluding the last two zeros, cannot be divided by four, i.e. 1700, 1800, 1900 and 2100). With the exception of a few rare mechanisms, century calendar watches have to be adjusted once every 400 years.
Moon phase watches reproduce the lunar month. In the case of simple moon phases, the length of a lunar month displayed on the dial is 29.5 days, although in reality a lunar cycle lasts for 29 days, 12 hours, 44 minutes and 2.8 seconds. This system therefore requires a correction roughly once every 2 years and 7 months. In the case of watches fitted with an astronomical moon phase, the length of the lunar month displayed on the dial is 29 days, 12 hours and 45 minutes, so that a correction of one day is needed just once every 122 years.
The time equation is the difference between the time displayed on the watch (mean time) and the real time. True time will be the real time based on the position of the sun. The difference in seconds exists and changes every day. Some watches are equipped with a device enabling the difference between mean time and real time to be displayed every day.
The times at which the sun rises and sets at any particular place can be indicated by dedicated hands on auxiliary dials.
Ephemerides are astronomical indications showing the position of the planets on each day of the year. Generally, these indications are displayed on rotating discs which represent the celestial vault.
Sidereal time is zero when the vernal point passes into the meridian plane at a particular place and increases by one sidereal hour each time the earth rotates by 15° in relation to the vernal point. Sidereal time is literally the time of the stars and not that of the sun. While the passage of the sun defines, between two successive middays, the average day of 24 hours, that of the stars defines a different day of just 23 hours and 56 minutes in solar time but 24 hours in sidereal time. Some particularly sophisticated watches display sidereal time.
Useful for fishermen and yachtsmen, tide measuring watches show the high and low tide times at a given latitude.