Answer

A replica Patek Philippe perpetual calendar moon phase watch copies the dial. It does not copy the movement. A genuine Patek Philippe perpetual calendar holds the day, date, month, leap year, and moon phase with no correction until Feb 28, 2100. Copies lose track at the end of any month with fewer than 31 days. The moon phase window on most copies hides a disc that turns once per day or once per 29.5 days. Repair shops see these watches with jammed calendar plates and stripped pusher threads.

What the real watch does

Patek Philippe builds perpetual calendar moon phase references on self-winding and micro-rotor calibers. Cal. 240 Q sits in the Ref. 3940 and Ref. 5140. Cal. 324 S Q sits in the Ref. 5327 and the Ref. 5496P. Cal. 31-260 REG QA sits in the Ref. 5236P. The gear train carries a 48-month cam. The cam steps once per month and adds one day at each February 29. Moon phase gearing uses a 135-tooth wheel with two moons. That ratio matches the 29.53-day synodic month to one day of error in 122 years.

Case sizes run from 36 mm on the Ref. 3940 to 41.3 mm on the Ref. 5236P. Cases come in yellow gold, rose gold, white gold, platinum, and steel. Platinum references carry a diamond between the lugs at 6 o'clock, not on the dial.

What the copies use

Most copies start with a stock automatic movement and bolt a calendar module on top. Common bases are the Seagull ST21, the Hangzhou 6300, and clones of the ETA 2892 and the Valjoux 7750. The base movement is a three-hand or chronograph caliber. It has no month cam and no leap year cam.

The module adds three or four subdials. It does not add a 48-month cam. The date disc holds 31 positions, so the owner corrects the date by hand on May 1, July 1, Oct 1, and every other 30-day month, plus February. Leap years get no correction path at all. Push pins on the case side at 8, 9, and 10 o'clock drive the day, date, and month. Those pins have no detents on low-cost copies and strip after a few dozen uses.

How the moon phase is faked

Real moon phase discs are brass or gold and carry two painted moons. The disc makes one turn in 29 days, 12 hours, 44 minutes. Replica discs are stamped from thin sheet with one or two moons. Gearing falls into three groups. The first group copies the 59-tooth layout and is close to correct. The second uses a single moon on a fixed 29-day or 30-day gear, so it drifts by a day every few months. The third, most common on watches under $300, drives the disc from the 24-hour wheel. That disc completes a full turn each day. It is a day and night indicator, not a moon phase.

Dial and case details to check

• Subdial placement. Ref. 3940 and Ref. 5140 put the day and month at 9 and 3 o'clock. Date and moon phase sit at 12 and 6. Copies shift these by up to 1 mm.
• Markers. Genuine dials use applied gold faceted markers. Copies use glued or printed markers with flat tops.
• Caseback. Ref. 3940 has a solid back with an engraved reference and movement number. Ref. 5140 has a sapphire display back over Cal. 240 Q.
• Weight. Platinum and gold cases weigh more than steel. Brass and zinc alloy cases weigh less than both.
• Printing. Genuine dial text is pad printed and sharp at the serifs. Copy dial text shows ink spread and misaligned lines.

Price and market

Replica perpetual calendar moon phase watches sell from about $150 to $900. The low tier uses quartz or a 21,600 vph automatic with a printed dial and mineral glass. The upper tier uses a 28,800 vph clone movement and a sapphire crystal. Genuine Patek Philippe perpetual calendars in gold trade from about $30,000 for a Ref. 3940 to six figures for a Ref. 2499. That gap is the reason for the copy market.

Legal status in the US

Counterfeit watches are illegal to sell and to import in the United States. 18 U.S.C. § 2320 allows up to 10 years in prison for trafficking in counterfeit goods. U.S. Customs and Border Protection opens international mail and express consignment parcels and destroys counterfeit watches found inside. A buyer who loses a parcel to seizure has no refund path. Sellers face civil suits from the trademark holder.

Bottom line

No replica on the market runs a true perpetual calendar. The moon phase on most copies is decorative or a 24-hour disc. Treat any listing that claims a full perpetual calendar and leap year function as false.