Basis is a multiplier used to calculate the total interest paid or received in a defined time period. It is typically used in accounting and valuation for bonds, swaps, dividends, project finance, and other investments with periodic cash flows. Basis is also known as the "day count convention" because it defines how days in a year are treated for calculation purposes in different parts of the world.

Eurobond
The calculation for this basis is similar to that for 30/360E, with one notable difference. For Eurobond all months are considered to be 30 days long, irrespective of their actual calendar length. However, if the termination date of the final period is the last day of February, the actual number of days in the month (28 or 29) is used instead for this period. The Eurobond basis is calculated as follows: 30 days/month N = (D2 - D1) + 30 x (M2 - M1) + 360 x (Y2 - Y1) Where: D1, M1, Y1 = day, month and year of the earlier date D2, M2, Y2 = day, month and year of the later date