Conventional watchmaking has solved the problem of telling time with two hands on a circular dial. Everything else is an attempt to do the same thing differently, and those attempts now number in the dozens. I collectively call them «unconventional time display» because they question not the precision of the movement, but the way the result reaches the eye.
The oldest line is that of wandering hours, documented in the 17th century: discs with hour numerals scroll across a minute scale instead of staying fixed at the center. Audemars Piguet miniaturized it in the 1990s with the Star Wheel patent, where transparent discs carry the numerals and rotate over a 120-degree arc. The principle is the same: the number comes to you, you don't circle around it with your eyes.
Urwerk built an entire identity on satellites: hour blocks mounted on Geneva crosses, brought in sequence in front of the minute scale by a rotating carousel. In the UR-120 each satellite was broken down into two sub-elements to reduce thickness and improve readability; in the UR-220 the blocks are in solid Super-LumiNova, with a nighttime effect that's hard to ignore. The logic remains that of wandering hours, but the construction is 21st-century manufacture.
HYT introduced a completely different principle in 2012: two immiscible liquids in a glass capillary curved around the dial. The meniscus between colored fluid and transparent fluid indicates the hour; two bellows at 6 o'clock regulate the flow. It's a mechanical system in every respect, even though it has no gear wheels in the visible part. The past is on one side, the future on the other, the present is the junction.
The Kudoke 3 uses three arms of different lengths that alternate between two levels of the dial: hours readable on the lower plane on a triple scale, minutes on the upper ring. At 2, 6 and 10 o'clock two arms are visible simultaneously during the change. The complication is mechanical but reading it requires a moment of adjustment.
The butterfly or papillon complication, associated with Daniel Roth and taken up by Bulgari, uses two trapezoidal indicators that alternate on a semicircular scale: when one enters, the other exits. Different again is the displaced time display of the Dwiss M3, which arranges hours, minutes and seconds on separate concentric rings with the principle that the slowest indication is at the edge and the fastest at the center.
What unites all these systems is a real mechanical cost: more parts, more friction, more points of possible drift over time. Readability is almost always inferior to a well-proportioned classic hand. The value is not functional, it's conceptual. Those who make them know it, those who buy them know it. At the bench, when someone brings them to me for a service, the first thing I do is find out if the original manufacturer still exists, because for many of these movements there is no alternative.