The IWC Portugieser Eternal Calendar IW505701 is a conventional four-year perpetual calendar with an additional 400-year mechanism. That extra wheel observes the Gregorian calendar’s skipped leap years in 2100, 2200 and 2300—corrections the 2024 Portugieser Perpetual Calendar 44 cannot make automatically. The Eternal also extends calculated moon-phase accuracy from 577.5 years to 45 million years, while retaining IWC’s crown-adjusted, synchronized display architecture.
The essential difference: a four-year program versus a 400-year Gregorian cycle
Both watches track date, day, month, leap-year timing and the moon phase automatically. A standard perpetual calendar is programmed around the repeating four-year leap-year cycle: February gains a 29th day every four years. That rule works for most years but fails at Gregorian century years that are divisible by 100 and not by 400.
The Eternal Calendar adds what IWC calls a secular perpetual mechanism: an eight-part wheel that completes one revolution every 400 years. It tells the calendar to omit the leap day in 2100, 2200 and 2300, then retain it in 2400. The watch therefore extends ordinary perpetual-calendar logic to the Gregorian exceptions.
| Feature | Portugieser Eternal Calendar IW505701 | Portugieser Perpetual Calendar 44 (2024) |
|---|---|---|
| Calendar program | Four-year program plus a 400-year Gregorian exception wheel | Four-year perpetual-calendar program; skipped century leap years require correction |
| First relevant exception | Automatically skips the leap day in 2100 | Needs a minor adjustment for 2100, and similarly for 2200 and 2300 |
| Moon-phase indication | Calculated to deviate by one day after 45 million years | Calculated for 577.5 years before a one-day deviation |
| Movement | IWC calibre 52640 | IWC calibre 52616 |
| Power reserve | 168 hours (seven days) | Seven days |
| Case options | Platinum | Four references in 18-carat white gold or Armor Gold |
| Adjustment method | Crown advancement, with the 400-year exceptions calculated mechanically | Crown advancement; century exceptions outside the normal leap-year rule must be corrected |
How the Eternal Calendar handles 2100
Under the Gregorian calendar, a century year is a leap year only when it is divisible by 400. Thus 2000 had a February 29, but 2100, 2200 and 2300 will not. The Eternal Calendar’s mechanism is designed to make those decisions without the owner intervening.
Recommended Free Tools
#1 Best Overall
- Model IW391031
- The normal four-year calendar program advances the date, month and leap-year cycle.
- The 400-year wheel tracks the position within the Gregorian cycle.
- At 2100, 2200 and 2300, the wheel suppresses the otherwise expected February 29.
- At 2400, the leap day returns because 2400 is divisible by 400.
IWC says the mechanism is designed to calculate correctly at least through 3999. The Gregorian status of the year 4000 has not been officially decided, so the watch’s stated horizon stops short of claiming an answer for that year.
“Our first secular perpetual calendar automatically takes into account the Gregorian calendar’s complex leap-year exception rules by skipping the leap year three times over 400 years.”
— Chris Grainger-Herr, CEO of IWC Schaffhausen, 9 April 2024
Rank #2
Is the 45-million-year moon phase accurate?
“45 million years” is IWC’s calculated interval before the Double Moon display accumulates a one-day error; it is not a guarantee about the watch’s service life or a laboratory observation over that timespan. IWC says engineers simulated more than 22 trillion wheel combinations when developing the reduction gear.
Windows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallCrashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteThe display uses two superimposed moon discs for the Northern and Southern hemispheres. A reduction gear converts the calendar month into the Moon’s synodic period of 29 days, 12 hours, 44 minutes and 2.88 seconds. The result is a dramatic increase over the 577.5-year calculated accuracy of the Perpetual Calendar 44, but both figures describe mathematical drift rather than a practical ownership limit.
Case, movement and display specifications
Eternal Calendar IW505701
| Case | Platinum |
|---|---|
| Diameter | 44.4 mm |
| Height | 14.9 mm |
| Water resistance | 5 bar |
| Dial and strap | White dial; black Santoni alligator strap |
| Movement | Calibre 52640, automatic winding |
| Reserve and frequency | 168 hours; 28,800 vibrations per hour (4 Hz) |
| Construction | 396 components and 54 jewels |
| Indications | Date, day, month, four-digit year and Double Moon display |
Perpetual Calendar 44
The 2024 Perpetual Calendar 44 is offered in four references—IW503701, IW503702, IW503703 and IW503704—with combinations of 18-carat white gold or Armor Gold cases and different dials. All use calibre 52616, a seven-day power reserve and a moon-phase indication calculated to remain accurate for 577.5 years. The researched specifications do not establish a single case diameter, thickness or water-resistance figure for every reference, so those details should be checked against the specific model.
Rank #3
- Silicone watch strap soft and dust-resistant , comes with a quality stainless steel buckle, which is made of stainless steel.
- Please measure the width of your watch lug before ordering. It is important that your watch's lug width is the exact width of the band you select.
- Softness is moderate, wear very comfortable,Compression molding, sturdy and ,Wearable strap/wrist strap,suitable for original watch.
- Comfortable Fit: Soft, skin - friendly rubber conforms to wrist curves. Multiple adjustment holes ensure a perfect fit for all wrist sizes, offering stable, non - tight wear.
- Easy to insert and remove,very suitable for your watch.
What both watches share in daily use
IWC uses a crown-adjusted, synchronized calendar architecture in both designs. Advancing the crown moves the calendar indications together rather than requiring separate correctors for each display. Both also use the Double Moon concept, showing the two hemispheres on paired discs.
The visual treatment is deliberately mechanical: subdials present the calendar information, while a glass dial, double box-glass sapphire crystals and views of the movement expose the layered display and gear train. The Eternal’s additional 400-year wheel changes the calendar’s logic, not the owner’s basic crown-based method of setting it.
Free tools Windows power users keep installed
One-click scans. No signup required.
Which Portugieser calendar should you buy?
Choose the Eternal Calendar if the 2100 problem matters
- You want a calendar mechanically programmed for Gregorian century exceptions rather than a conventional four-year perpetual cycle.
- You value the 45-million-year calculated moon-phase figure and the technical story behind its 22-trillion-combination simulation.
- You specifically want the IW505701’s platinum case, 44.4 mm diameter and calibre 52640.
Choose the Perpetual Calendar 44 if the established design is enough
- You prefer one of the four 2024 white-gold or Armor Gold references.
- A 577.5-year calculated moon-phase accuracy already exceeds any practical ownership horizon.
- You accept that a skipped century leap year such as 2100 will require a minor crown adjustment.
Price and availability
A current retail price, stock position and regional availability are not established by the specifications above. Check IWC Schaffhausen or an authorized retailer for the exact reference, market and date before purchasing.
Quick Recap
Video walkthrough: details worth watching
- Start with the date, day, month, four-digit year and Double Moon indications on the wrist.
- Look for the explanation of the normal four-year month cam and the added 400-year wheel.
- Compare the Eternal’s 45-million-year calculated moon-phase interval with the Perpetual 44’s 577.5-year figure.
- Examine the platinum IW505701 beside the Perpetual 44’s gold case options, along with the glass dial, sapphire crystals and movement views.
- Note that both are crown-adjusted, but only the Eternal is mechanically programmed for the Gregorian century exceptions.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
