Perpetual versus annual calendar
A standard date display usually needs correction after months with fewer than 31 days. An annual calendar accounts for 30- and 31-day months but generally needs one correction each year after February. A perpetual calendar is more complex: it is designed to recognize the ordinary month sequence and leap years automatically.
Calendar terms are not always used consistently in marketing, so check the exact watch's technical description. Some watches combine the calendar with moon phase, day, month, leap-year indication or other complications; the number and layout of indications differ by caliber.
Why setting instructions matter
The calendar mechanism stores a program of dates and shifts its indications around midnight. Quick-setting during a restricted period can interfere with the mechanism on some movements. The safe setting window and crown sequence vary; there is no universal rule for every caliber.
If the watch stops, the calendar may need extensive correction. A watchmaker or the manufacturer's instructions can help avoid incorrect setting. For a high-complication watch, keep the manual and have the seller demonstrate the setting process before purchase.
A beautiful complication with real trade-offs
Perpetual calendars demonstrate mechanical memory and miniaturization, but extra complexity can increase purchase price, service needs and the consequences of mishandling. Consider whether you value the calendar display and mechanical ingenuity enough to accept those responsibilities.
There are electronic and hybrid ways to manage calendars, and many owners prefer a simpler date or annual-calendar watch. Complication count is not a quality ranking: legibility, reliability, serviceability and thoughtful execution matter just as much.