360-day Calendar

The 360-day calendar is a method of measuring durations used in financial markets, in computer models, in ancient literature, and in prophetic literary genres. It is based on merging the three major calendar systems into one complex clock, with the 360-day year as the average year of the lunar and the solar: 365.24 (solar) + 354.37(lunar) = 719.61 ÷ 2 = 359.8 days rounded to 360. It is a simplification to a 360-day year, consisting of 12 months of 30 days each. To derive such a calendar from the standard Gregorian calendar, certain days are skipped.

A duration is calculated as an integral number of days between two dates A and B (where by convention A is earlier than B). There are two methods commonly available which differ in the way that they handle the cases where the months are not 30 days long:

• The European Method (30E/360)
  • If either date A or B falls on the 31st of the month, that date will be changed to the 30th;
  • Where date B falls on the last day of February, the actual date B will be used.

• The US/NASD Method (30US/360)
  • If both date A and B fall on the last day of February, then date B will be changed to the 30th.
  • If date A falls on the 31st of a month or last day of February, then date A will be changed to the 30th.
  • If date A falls on the 30th of a month after applying (2) above and date B falls on the 31st of a month, then date B will be changed to the 30th.

In both cases the difference between the possibly-adjusted dates is then computed by treating all intervening months as being 30 days long.

Other methods include the ISDA 360-day calendar, and the PSA 360-day calendar.