Perpetual Calendar

From Chinese Watch Industry Wiki

A perpetual calendar is a calendar mechanism which automatically corrects for short and long months and leap years. By contrast, a conventional calendar mechanism counts 31 days for every month and thereby needs periodic correction. Perpetual calendars are commonplace in electronic watches and are becoming more common is quartz analogue watches, however the complexity required to achieve the same result in a mechanical watch is such that mechanical perpetual calendar watches remain very rare and expensive.

The only mechanical perpetual calendar currently manufactured in China is the Sea-Gull ST2590.