The Excel DISC function returns the discount rate for a security. A discounted security does not pay periodic interest, but has a specified redemption value at maturity. The return on a discounted security is the difference between the price and redemption value.
Discount rate is based on the concept of the time value of money. The discount rate is the interest used to convert between future value and present value. A future value can be "discounted" by a given interest rate to determine the present value.
In the example shown, we want find the discount rate for a bond with a price of $89.50 and a redemption value of $100. The settlement date is 6-Jul-2017 and the maturity date is 15-Jan-2020. The day count basis is US (NASD) 30/360. The formula in F5 is:
With these inputs, the DISC function returns 4.16%, with percentage number format applied.
In Excel, dates are serial numbers. Generally, the best way to enter valid dates is to use cell references, as shown in the example. To enter valid dates directly inside a function, you can use the DATE function. To illustrate, the formula below has all values hardcoded, and the DATE function is used to supply each of the two required dates:
The basis argument controls how days are counted. The DISC function allows 5 options (0-4) and defaults to zero, which specifies US 30/360 basis. This article on wikipedia provides a detailed explanation of available conventions.