A quick look at a romantic rather than practical complication on a watch, the Equation of Time.
Definition as per the dictionary: “The difference between apparent time and mean time usually expressed as a correction which is to be added to apparent time to give local mean time.” What the dictionary is trying to tell you, is that there’s a difference between two measures of time. There’s a difference between the solar time and the time shown on our clocks and watches. This equation of time equals the apparent solar time minus the mean solar time, the positive or negative result of that is then shown on the watch.
Therefore, the equation of time complication essentially shows the discrepancy between the true solar time (what you see on a sundial), and mean solar time (what you see on your watch). The difference, or ‘deviation of time’ between both can run between minus 16 minutes and plus 14 minutes (more or less), and the little gauge on the dial shows you precisely that deviation.
Not the most useful complications, yet certainly one of the more fascinating ones. Not to mention, this can make for one heck of a party trick.