NASD method

In the NASD method for calculating the number of days between two dates, if the day value in the starting date (for example, the settlement date) is 31, it is treated as if it were 30. If the day value is the last day of February, it is not adjusted, so in this case February has fewer than 30 days. If the day value for the ending date (for example, the maturity date) is 31 and the day value in the starting date is earlier than the 30th of the same month, the ending date is considered to be the first day of the following month. Otherwise it is considered to be the 30th of the same month, resulting in 0 days.

By comparison, in the European method, the 31st of a month is always considered to be the 30th of the same month. February is always considered to have 30 days, so if the last day of February is the 28th, it is considered to be the 30th.