code: public static function TBILLEQ($settlement, $maturity, $discount)
      {
          $settlement = Functions::flattenSingleValue($settlement);
          $maturity = Functions::flattenSingleValue($maturity);
          $discount = Functions::flattenSingleValue($discount);
  
          //    Use TBILLPRICE for validation
          $testValue = self::TBILLPRICE($settlement, $maturity, $discount);
          if (is_string($testValue)) {
              return $testValue;
          }
  
          if (is_string($maturity = DateTime::getDateValue($maturity))) {
              return Functions::VALUE();
          }
  
          if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_OPENOFFICE) {
              ++$maturity;
              $daysBetweenSettlementAndMaturity = DateTime::YEARFRAC($settlement, $maturity) * 360;
          } else {
              $daysBetweenSettlementAndMaturity = (DateTime::getDateValue($maturity) - DateTime::getDateValue($settlement));
          }
  
          return (365 * $discount) / (360 - $discount * $daysBetweenSettlementAndMaturity);
      }
docs: TBILLEQ.
  
  Returns the bond-equivalent yield for a Treasury bill.
  
  @param mixed $settlement The Treasury bill's settlement date.
  The Treasury bill's settlement date is the date after the issue date when the Treasury bill is traded to the buyer.
  @param mixed $maturity The Treasury bill's maturity date.
  The maturity date is the date when the Treasury bill expires.
  @param int $discount The Treasury bill's discount rate
  
  @return float