The COUPDAYBS function calculates the number of days from the beginning of a coupon period to the settlement date. The "BS" in COUPDAYSBS means "Beginnining to Settlement".

COUPDAYBS takes 3 required arguments and 1 optional argument:

Syntax: COUPDAYBS(settlement, maturity, frequency, [basis])

The settlement date is the date a buyer purchases a coupon, such as a bond. The maturity date is the date when a coupon expires. For example, suppose a 10-year bond is issued on January 1, 2014, and is purchased by a buyer seven months later. The issue date would be January 1, 2014, the settlement date would be August 1, 2014, and the maturity date would be January 1, 2024, 10 years after the January 1, 2014, issue date.

##### Using the COUPDAYBS function:
The arguments for the COUPDAYBS function are:
Argument Required? Description
settlement Required The security's settlement date. The security settlement date is the date after the issue date when the security is traded to the buyer.
maturity Required The security's maturity date. The maturity date is the date when the security expires.
frequency Required The number of coupon payments per year. For annual payments, frequency = 1; for semiannual, frequency = 2; for quarterly, frequency = 4.
basis Optional The type of day count basis to use.
##### A few more things:
 • Excel stores dates as sequential serial numbers so they can be used in calculations. By default, January 1, 1900 is serial number 1, and January 1, 2014 is serial number 41640 because it is 41,640 days after January 1, 1900. • All arguments are truncated to integers. • If settlement or maturity is not a valid date, COUPDAYBS returns the #VALUE! error value. • If frequency is any number other than 1, 2, or 4, COUPDAYBS returns the #NUM! error value. • If basis < 0 or if basis > 4, COUPDAYBS returns the #NUM! error value. • If settlement ≥ maturity, COUPDAYBS returns the #NUM! error value.

### Summary

The COUPDAYBS function calculates the number of days from the beginning of a coupon period to the settlement date.