The 360 day calendar is a method of measuring durations used in financial markets. It is based on the assumption of a 360 day year, consisting of 12 months of 30 days each. To arrive at such a calendar from the standard 365/366 day Gregorian calendar, certain days are skipped.

There are two stages in calculating the difference between two dates A and B, with B later than A. First, if A or B occur on the 31st of the month, they must be replaced by a substitute date not lying on the 31st of the month. There are two methods of doing this: the US or NASD method, and the European method.

The European method is the simplest: in the European method, if either date lies on the 31st of the month, replace it with the first of the next month.

The US/NASD method is more complex. If date A is the 31st of the month, replace it with the 30th of the month. If date B is the 31st of the month, and date A is before the 30th of the month, then date B becomes the 30th of the month, else date B becomes the 1st of the next month. Why does the US use this more complex definition? Are there any advantages to it?

The US/NASD method is defined in: Securities Industry Association , Standard Securities Calculation Methods: Fixed Income Securities Formulas for Analytic Measures, Vol. 2, Spring 1995.

An alternative form of the US/NASD has been defined (not industry standard), in which dates A and B are treated identically. This is made available by the Gnumeric spreadsheet package.

Reportedly, Excel's US/NASD method is not SIA-compliant.

Other methods include the ISDA 360-day calendar, and the PSA 360-day calendar. Information on these?

The 360 day calendar is implemented by:

This should not be confused with the 360 calendar .