The COUPDAYS function in Excel returns the number of days in the coupon period that contains the settlement date of a bond.In other words, it tells you how many days are in the full coupon period (between two coupon payments) where your purchase date falls.

The COUPDAYBS function in Excel calculates the number of days from the beginning of the coupon period to the settlement date of a bond. It’s mainly used when working with bonds and fixed-income securities that pay periodic interest (coupons).