Using a watch to tell the time may sound incredibly basic, but it's actually quite complex. There are many different ways in which time can be displayed that it is worth pointing out the obvious in this case.
Watches that tell the time using hands are known as analog watches. The most standard handset (family of hands) you will find on a watch will comprise of an hour, a minute, and a seconds hand. The hour is generally short and stout, the minute long and slightly narrower, and the seconds hand tends to be shaped like a needle, often with a more prominent tip like an arrow or dot referred to as a lollipop hand.
Time can also be told digitally with just the use of numbers, similar to an alarm clock. Many of these digital watches are battery powered, though some are mechanically driven with their time regulated by a quartz crystal.
Functions of a watch beyond telling time are called complications. A popular complication is to tell the date, though it is not universal across all watches. Depending on the type of watch, adding a date function can make a lot of sense. For instance, a date complication complements a GMT watch quite naturally. However, in other styles, like a dive watch, this type of functionality doesn't make as much sense given the purpose of the watch.
The date complication can be placed anywhere on the watch, assuming the watchmaker adjusts the movement layout accordingly. A popular location for dial cut-out or window displays is located at the 3 o'clock or 6 o'clock positions, or between the 4 and 5 o'clock. The date can also be indicated by way of a hand, either pointing to a number printed around the outside of the dial or contained within a “sub-dial,” a separate miniature dial where additional information can be displayed.
Similar to the date complication, some watches also display the day of the week. It is common for day discs to be printed in two languages, with English and French being the most common options. It is also normal to see the abbreviation of Sunday (SUN) colored in red or blue.
The day complication is normally displayed at either the 3 o'clock or 6 o'clock position, due to the added space and reduced flexibility of the function. Less commonly, as with the Junghans Meister Calendar, it is displayed in a window of its own. It is also possible to display the day of the week in a sub-dial, but this is also quite seldom seen.
A GMT function enables the wearer to simultaneously display two time zones on their watch at once. A simple GMT function uses one extra hand that is geared to run at half the speed of the hour hand, so it is running on a 24-hour cycle (resulting in one rotation of the dial per day as opposed to two full rotations that the hour hand would normally make). A secondary benefit to a GMT hand is that if it is synchronized with your normal home time, it works as an AM/PM indicator.
There are also some options that show three time zones at once by utilizing a rotating external bezel as well as GMT, but reading the time that way is somewhat less intuitive.
The power reserve is a very simple complication mostly found on manually wound watches, which need to be rewound every day to prevent them from running out of power. The power reserve is essentially a fuel gauge, and will remind you to wind your watch when the power gets critically low should you forget. Power reserves are also featured on automatic watches, but are less essential due to the watch remaining fully wound thanks to the movement of the wearer’s wrist.
A moon phase indicator shows the phase of the moon as it waxes and wanes. This cycle takes roughly 29.5 days so it requires creative gearing to display accurately. Normally a moon phase disc has 59 teeth and two moons printed on it (so one full rotation of the disc accounts for two complete phases of the moon). The moon phase complication comes in all shapes and sizes, but it is most commonly contained within a sub-dial at 6 o’clock.
There is a big difference between entry-level moon phase complications and the very advanced technology going into some high-end pieces today. For the most part, a simple moon phase indicator is more than accurate enough for the casual wearer, but some of the very best are capable of remaining precise for more than a hundred years.