A leap year is a year which contains an extra days. It is used for keeping the Gregorian Calendar Year to synchronize with the Astronomical Year or Seasonal Year.

As astronomical events, and seasons do not repeat at an exact number of full days, a calendar that had the same number of days in each year would, over time, drift with respect to the event it was supposed to track.

By occasionally inserting (or intercalating) an additional day or month into a year, the drift can be corrected. Hence, there are 29 days in instead of 28 day in February in a leap year. Consequently, the leap year lasts 366 days instead of 365 days.

Girard-Perregaux, ww.tc - Financial

"ww.tc" is the abbreviation for World Wide Time Control. It is capable to show time in 24 different cities. Besides, it has the "Financial" indicators for the business hours of 4 major stock markets across the globe, i.e. New York, London, Hong Kong and Tokyo

An Astronomical Year (one revolution of the earth around the sun) is exactly 365 days, 5 hrs, 45 mins, and 46 secs, or mathematically expressed as 365.242192 days. To compensate for the slightly less than quarter of a day, every 4 year is a leap year under the Gregorian system.

However, every 4 year is a leap year will overcompensate by a few decimal places. Therefore, Feb 29th is ommited in every 100 years. Even with this correction, there is still a very small residue becomes undercompensated. To take care of this, the leap year is untouched in every 400 years. The rounding errors amount will lead to another exception in every 4000 years.

Franck Muller, Aeternitas Perpetual Calendar Chronograph, Ver. II

Unlike a conventional perpetual calendar (with a 1,416 days cycle or a 4 years cycle) which needs three manual correction in every 400 years (i.e. in 2100, 2200, and 2300; but not in 2400), Aeternitas Ver. II follows a 1,000 year cycle, and takes into the account of the the century years which are divisible by 400 (i.e. 2400, and 2800). Hence, Aeternitas Ver. II only needs one manual correction in every 4000 years.

On the dial, the moon phases are displayed with the utmost precision. The mistake is only 6.8 seconds per lunar month. It represents a mistake of only one day, and needs one manual correct every 1000 years, whereas in a traditional system the mistake is of one day every four years.

Therefore, Aeternitas Ver. II is also known as the "Eternal Calendar".

1992, 1996, 2004 and 2008 (evenly divisible by 4) are leap years. 1900, 2100, 2200, and 2300 are the 100 year exception (evenly divisible by 100), they are common years. 1600, 2000, 2400, and 2800 are 400 year exception of the 100 year exception (evenly divisible by 400), they are leap years again.

However, 4000 is the 4000 year exception. It is another common year (evenly divisible by 4,000).

As astronomical events, and seasons do not repeat at an exact number of full days, a calendar that had the same number of days in each year would, over time, drift with respect to the event it was supposed to track.

By occasionally inserting (or intercalating) an additional day or month into a year, the drift can be corrected. Hence, there are 29 days in instead of 28 day in February in a leap year. Consequently, the leap year lasts 366 days instead of 365 days.

Girard-Perregaux, ww.tc - Financial

"ww.tc" is the abbreviation for World Wide Time Control. It is capable to show time in 24 different cities. Besides, it has the "Financial" indicators for the business hours of 4 major stock markets across the globe, i.e. New York, London, Hong Kong and Tokyo

An Astronomical Year (one revolution of the earth around the sun) is exactly 365 days, 5 hrs, 45 mins, and 46 secs, or mathematically expressed as 365.242192 days. To compensate for the slightly less than quarter of a day, every 4 year is a leap year under the Gregorian system.

However, every 4 year is a leap year will overcompensate by a few decimal places. Therefore, Feb 29th is ommited in every 100 years. Even with this correction, there is still a very small residue becomes undercompensated. To take care of this, the leap year is untouched in every 400 years. The rounding errors amount will lead to another exception in every 4000 years.

Franck Muller, Aeternitas Perpetual Calendar Chronograph, Ver. II

Unlike a conventional perpetual calendar (with a 1,416 days cycle or a 4 years cycle) which needs three manual correction in every 400 years (i.e. in 2100, 2200, and 2300; but not in 2400), Aeternitas Ver. II follows a 1,000 year cycle, and takes into the account of the the century years which are divisible by 400 (i.e. 2400, and 2800). Hence, Aeternitas Ver. II only needs one manual correction in every 4000 years.

On the dial, the moon phases are displayed with the utmost precision. The mistake is only 6.8 seconds per lunar month. It represents a mistake of only one day, and needs one manual correct every 1000 years, whereas in a traditional system the mistake is of one day every four years.

Therefore, Aeternitas Ver. II is also known as the "Eternal Calendar".

1992, 1996, 2004 and 2008 (evenly divisible by 4) are leap years. 1900, 2100, 2200, and 2300 are the 100 year exception (evenly divisible by 100), they are common years. 1600, 2000, 2400, and 2800 are 400 year exception of the 100 year exception (evenly divisible by 400), they are leap years again.

However, 4000 is the 4000 year exception. It is another common year (evenly divisible by 4,000).

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