Quick answer

A fake Patek Philippe perpetual calendar usually fails on four checks: corrector positions in the case band, the leap year indicator, moonphase disc accuracy, and the spacing of the subdials. Most replica versions also do not run a true perpetual calendar at all. They use an annual calendar or a simple calendar module behind a printed leap year window, which means the watch needs manual correction after every February that is not part of a leap year.

What a real perpetual calendar does

The mechanism tracks months of 28, 30 and 31 days and inserts 29 February every four years. Patek Philippe caliber 240 Q, used in references such as the 3940 and 5140, drives day, date, month, moonphase and leap year from one gear train. Geometry does the work, not electronics. That matters when spotting fakes, because the printed information on the dial has to match the positions of the corrector pushers set into the case band.

Checks you can run in five minutes

  1. Count the correctors. A real perpetual calendar has recessed correctors in the case band for day, date, month and moonphase. Count them and note where they sit.
  2. Find the leap year aperture. It shows 1, 2, 3 and 4, or L, 1, 2, 3. If the window is printed on the dial with no moving disc behind it, it is decoration.
  3. Test the date change. Set the watch to 23:55 and watch the flip. It should snap once within a few minutes.
  4. Check subdial layout. On a 3940 the subdials sit at 3, 6, 9 and 12, with the moonphase at 6 and the month at 9. Two subdials at 6 and 12 in a four subdial arrangement points to a copied dial on a generic movement.
  5. Inspect the moonphase disc. The lunar disc should be silvered or gold toned, with sharp crater detail and a smooth rotating aperture. Flat printed discs are common on replicas.

Movement and caseback inspection

  1. Turn the watch over. References 3940, 5140 and 5320 use display backs, so the movement should be visible.
  2. Look for a micro-rotor. The 240 Q uses an off-center micro-rotor sunk into the bridges. A full size rotor or a solid caseback on a model that should have a display back is a warning sign.
  3. Check the finish. Geneva stripes, perlage and beveled edges are applied by hand. Stamped decoration looks flat under a loupe.
  4. Measure case thickness. Perpetual calendars carry a thick module. A case that is thinner than the reference specification is a rebuilt case holding a different movement.

Reference specific tells

  • 3940: 36 mm case, four subdials, correctors at roughly 8, 10, 12 and 2 hours.
  • 5140: 37.2 mm case, four subdials, a thicker bezel than the 3940.
  • 5320: three subdials, day and month in windows at 9 and 3, moonphase at 6 with a leap year window.
  • 5740 Nautilus: perpetual calendar in a sports case, day and month windows, moonphase at 6, integrated bracelet.

Before you buy

Ask for photographs of the corrector positions, a numbered leap year disc, and a movement shot through the caseback. A seller who will not provide these is not offering the watch described. Any listing that claims the word genuine while showing a printed leap year window is a misrepresentation, and selling a replica as an original is a criminal offense in the United States under 18 U.S.C. 2320. Replicas sold openly as replicas are legal to own in most US states, but they cannot be serviced by the brand and carry no manufacturer warranty.