The Excel financial function
COUPDAYS(settle,maturity,frequency,basis) returns the number of days in the coupon period in which settlement occurs.
I am wondering why does this function return a non-integer number of days when the basis is 3 (Actual/365)? E.g. A security will settle on April 2, 2020 (settle), matures on December 31, 2025 (maturity), and pays interest quarterly (frequency), then
COUPDAYS(2/4/2020,31/12/2025,4,3) = 91.25. For all the other basis 0,1,2,4 one gets, as expected, a result that is an integer value for the number of days in the coupon period.