A perpetual calendar watch has three or four subsidiary dials to indicate the month, date, day and moon phase. Some dials may incorporate additional information such as a leap-year indicator, day and night indicator and power reserve display. The leap-year indicator has a scale from one to four, with four designated as the leap year.

The moon phase display is probably the most distinctive feature of a perpetual calendar watch, although it isn't an exclusive element. It tracks the progression of visible changes in the moon's appearance, and shows the full moon twice a month. It's featured in an individual aperture or combined with one of the other features, such as the date or 24-hour indicator. Many models such as Vacheron Constantin's malte perpetual calendar don't have a moon phase.

Most styles have a sapphire-crystal glass at the back of the dial, displaying the intricate mechanisms inside and the painstaking level of precision involved in the watch-making process. There are usually three to four pin-setting buttons around the bezel for day, date, month, and moon- phase adjustments.

Trend and tips

Almost all perpetual calendar watches are designed for men, with buyers typically in their 40s or older, says Noel Ying-Fei Wong, deputy director of Sotheby's and head of the watches department. Most brands make the case from precious metals such as gold or platinum, contributing to the overall value of perpetual calendar watches. Wong says that most buyers are connoisseurs who already own several mechanical watches, which explains the older age group. He also notices a growing interest in this type of watches among younger men in their 30s. Some models combine the movement with other functions such as split-second, fly-back and chronograph dials.

Patek Philippe is renowned as one of the first makers of the perpetual calendar movement - its first was made in 1925, followed by the first series of perpetual calendar chronographs in 1941, and the first automatic perpetual calendar movement in 1962. The complexities involved in the movement meant watch-makers could make them only by special order or in limited numbers.

Prices for a perpetual calendar watch generally range from $150,000 to several hundred thousands, depending on the brand, the make and whether there are other complications. Wong says most people prefer established brands such as Patek Philippe, Breguet and Vacheron Constantin, which are assurances

of quality and command higher

resale values.

Don't wear a perpetual calendar watch for sports. It may be water resistant, but precious metals can be scratched easily. If you don't wear it often, take it out to wind at least once a month. Consider buying a watch-winder with electronic timer to ensure your perpetual calendar watch is wound regularly, especially if you don't have the time to do so. Patek Philippe's automatic perpetual calendar watches come with a winder.

Shopping list

Blancpain

Available at Carlson Watch Co,

Shop G8, China Building, Central,

tel: 2525 5478; for other stockists,

tel: 2510 5121.

Its limited edition Le Brassus comes with a cigar cutter and unique dial.

Glashutte Original

Available at Elegant Watch Company, Shop 3, Luk Hoi Tong Building, Central, tel: 2868 1882; for stockists, tel: 2510 5142.

The award-winning Senator perpetual calendar features a ground-breaking design. Day-date-month, moon phase, and leap-year indicators are displayed through individual apertures.

IWC

Shop G29, G/F, Prince's Bldg, Central, tel: 2525 5323.

IWC's Portuguese perpetual calendar is ultra chic with a four-digit year display and is mechanically programmed.

Patek Philippe

Available at Masterpiece by King Fook Jewellery at Shop 21, Central Building, Central, tel: 2526 6733; for other stockists, tel: 2317 7866.

The latest perpetual calendar retains the brand's signature look with a sleeker design.