TBILLEQ function returns the bond-yield equivalent (BEY) for a treasury bill. TBILLEQ is equivalent to (365 x discount) / (360 - discount x DSM) where DSM is the days between @settlement and @maturity.
If @settlement is after @maturity or the @maturity is set to over one year later than the @settlement, TBILLEQ returns NUM! error. If @discount is negative, TBILLEQ returns NUM! error.