Short answer
A fake Patek Philippe perpetual calendar copies a reference such as the 3940, 5140, 5320G, 5236P or 5740/1G but uses a different movement. On most fakes the calendar is a module stacked on a base caliber. The module counts 28 or 31 days and starts over. A real Patek Philippe perpetual calendar runs from one gear train and tracks leap years until 2100.
What the real watch does
A perpetual calendar shows day, date and month. Most references add a leap year indicator. Some add a 24 hour dial, a moon phase or a chronograph. The display moves at midnight. In a leap year the watch goes from Feb 28 to Feb 29 on its own. The next correction for any Patek Philippe perpetual calendar falls in 2100, which is not a leap year.
Checks on the calendar
- Leap year hand. Advance the date with the corrector. On a real watch the indicator steps one position per year and returns after four years.
- Month change. Set the date to Feb 28 and wait. A 31 day module jumps to March 1 in a leap year.
- Moon phase. Patek moon phase gearing runs on a 59 tooth wheel. The stated error is one day in 122 years.
- Subdial spacing. Subdials sit at fixed distances from the center on each reference. Modules from other makers shift the spacing.
Movement
Reference 3940 and 5140 use caliber 240 Q, a micro rotor automatic with 27 jewels, about 275 parts and a power reserve near 48 hours. The 5320G uses caliber 324 S Q. The 5236P uses caliber 31 260 PS QL. Look at the caseback. A real movement shows Geneva stripes, beveled bridges, gold chatons and the Patek Philippe Seal. Fakes show a plain rotor or a printed plate.
Case size
Case size is a fast filter. The 3940 measures 36 mm. The 5140 measures 37.2 mm. The 5320G measures 40 mm. The 5236P measures 41.3 mm. Fake cases sit 1 mm to 2 mm larger than the reference they copy.
Price floor
Secondary prices for a Patek Philippe perpetual calendar start near $60,000. A 3940 in yellow gold trades at $45,000 to $70,000. The 5740/1G in white gold runs past $150,000. Counterfeits sell for $200 to $1,500. No seller offers a real 3940 for $900.
Papers and numbers
Patek Philippe issues an Extract from the Archives for watches made since 1839. The extract lists the movement number, the case number and the sale date. Match those numbers to the engraving inside the caseback. A counter can engrave numbers, so check font and depth with a loupe.
Limits
No photo check replaces hands on inspection. A watch with a real dial and a swapped movement is hard to spot on a screen. Buy from a dealer with a return policy, or send the watch to Patek Philippe for service. Patek Philippe does not service movements with counterfeit parts. Import of counterfeit watches is illegal in the United States, and customs can seize a shipment.