# Perpetual Calendar Simulator: Interactive Watch Complication
The perpetual calendar is one of the most prestigious complications in haute horlogerie. This interactive simulator visualizes how a mechanical perpetual calendar tracks date, day, month, leap year, and moon phase — all without manual correction for decades. Explore the 48-month gear program, see how February transitions work, and understand the gregorian calendar logic built into these masterpieces of micro-mechanics.# How a Perpetual Calendar Works
A mechanical perpetual calendar uses a program wheel with notches of different depths representing months of different lengths. A sensing lever drops into each notch; a deeper notch signals a short month (28-29 days), triggering the mechanism to advance to the 1st of the next month after the correct number of days. A 48-month gear handles the 4-year leap year cycle, with an extra notch for February 29th. The program knows that century years (e.g., 2100) skip leap year unless divisible by 400.# Calendar Logic Reference
| Month | Days | Notch Depth | Leap Year Action |
|---|---|---|---|
| January | 31 | Deep | Normal |
| February | 28 / 29 | Shallowest | Extra day every 4 years |
| March | 31 | Deep | Normal |
| April | 30 | Medium | Normal |
| May | 31 | Deep | Normal |
| June | 30 | Medium | Normal |
| July | 31 | Deep | Normal |
| August | 31 | Deep | Normal |
| September | 30 | Medium | Normal |
| October | 31 | Deep | Normal |
| November | 30 | Medium | Normal |
| December | 31 | Deep | Normal |
# Perpetual vs Annual Calendar
| Feature | Annual Calendar | Perpetual Calendar |
|---|---|---|
| Requires adjustment | Once per year (Mar 1) | Once per century (2100) |
| Leap year handling | Manual | Automatic (4-year cam) |
| Month recognition | 30 vs 31 days | Full 28/29/30/31 |
| Complexity | Moderate (~50 parts) | Very high (~200+ parts) |
| Price range | $3,000 - $15,000 | $20,000 - $500,000+ |
Interactive Calendar Simulator
This tool simulates the calendar mechanism of a perpetual calendar watch. The animated dial shows the date hand, month window, day subdial, moon phase, and leap year indicator. Use the controls to advance through days, months, or years and observe the mechanical logic in action.