diff --git a/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Cumulative.php b/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Cumulative.php
index 1e05a446..4f6ed170 100644
--- a/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Cumulative.php
+++ b/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Cumulative.php
@@ -4,6 +4,7 @@ namespace PhpOffice\PhpSpreadsheet\Calculation\Financial\CashFlow\Constant\Perio
 
 use PhpOffice\PhpSpreadsheet\Calculation\Exception;
 use PhpOffice\PhpSpreadsheet\Calculation\Financial;
+use PhpOffice\PhpSpreadsheet\Calculation\Financial\Securities\Constants;
 use PhpOffice\PhpSpreadsheet\Calculation\Functions;
 
 class Cumulative
@@ -30,7 +31,7 @@ class Cumulative
      *
      * @return float|string
      */
-    public static function interest($rate, $periods, $presentValue, $start, $end, $type = 0)
+    public static function interest($rate, $periods, $presentValue, $start, $end, $type = Constants::END_OF_PERIOD)
     {
         $rate = Functions::flattenSingleValue($rate);
         $periods = Functions::flattenSingleValue($periods);
@@ -51,7 +52,7 @@ class Cumulative
         }
 
         // Validate parameters
-        if ($type !== 0 && $type !== 1) {
+        if ($type !== Constants::END_OF_PERIOD && $type !== Constants::BEGINNING_OF_PERIOD) {
             return Functions::NAN();
         }
         if ($start < 1 || $start > $end) {
@@ -61,7 +62,7 @@ class Cumulative
         // Calculate
         $interest = 0;
         for ($per = $start; $per <= $end; ++$per) {
-            $ipmt = Financial::IPMT($rate, $per, $periods, $presentValue, 0, $type);
+            $ipmt = Interest::payment($rate, $per, $periods, $presentValue, 0, $type);
             if (is_string($ipmt)) {
                 return $ipmt;
             }
diff --git a/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Interest.php b/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Interest.php
index 1bb63a2a..3f579ce2 100644
--- a/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Interest.php
+++ b/src/PhpSpreadsheet/Calculation/Financial/CashFlow/Constant/Periodic/Interest.php
@@ -23,12 +23,12 @@ class Interest
      * Excel Function:
      *        IPMT(rate,per,nper,pv[,fv][,type])
      *
-     * @param float $interestRate Interest rate per period
-     * @param int $period Period for which we want to find the interest
-     * @param int $numberOfPeriods Number of periods
-     * @param float $presentValue Present Value
-     * @param float $futureValue Future Value
-     * @param int $type Payment type: 0 = at the end of each period, 1 = at the beginning of each period
+     * @param mixed $interestRate Interest rate per period
+     * @param mixed $period Period for which we want to find the interest
+     * @param mixed $numberOfPeriods Number of periods
+     * @param mixed $presentValue Present Value
+     * @param mixed $futureValue Future Value
+     * @param mixed $type Payment type: 0 = at the end of each period, 1 = at the beginning of each period
      *
      * @return float|string
      */
@@ -81,13 +81,10 @@ class Interest
      * Excel Function:
      *     =ISPMT(interest_rate, period, number_payments, pv)
      *
-     * interest_rate is the interest rate for the investment
-     *
-     * period is the period to calculate the interest rate.  It must be betweeen 1 and number_payments.
-     *
-     * number_payments is the number of payments for the annuity
-     *
-     * pv is the loan amount or present value of the payments
+     * @param mixed $interestRate is the interest rate for the investment
+     * @param mixed $period is the period to calculate the interest rate.  It must be betweeen 1 and number_payments.
+     * @param mixed $numberOfPeriods is the number of payments for the annuity
+     * @param mixed $principleRemaining is the loan amount or present value of the payments
      */
     public static function schedulePayment($interestRate, $period, $numberOfPeriods, $principleRemaining)
     {
@@ -189,17 +186,17 @@ class Interest
         return $close ? $rate : Functions::NAN();
     }
 
-    private static function rateNextGuess($rate, $nper, $pmt, $pv, $fv, $type)
+    private static function rateNextGuess($rate, $numberOfPeriods, $payment, $presentValue, $futureValue, $type)
     {
         if ($rate == 0) {
             return Functions::NAN();
         }
-        $tt1 = ($rate + 1) ** $nper;
-        $tt2 = ($rate + 1) ** ($nper - 1);
-        $numerator = $fv + $tt1 * $pv + $pmt * ($tt1 - 1) * ($rate * $type + 1) / $rate;
-        $denominator = $nper * $tt2 * $pv - $pmt * ($tt1 - 1) * ($rate * $type + 1) / ($rate * $rate)
-            + $nper * $pmt * $tt2 * ($rate * $type + 1) / $rate
-            + $pmt * ($tt1 - 1) * $type / $rate;
+        $tt1 = ($rate + 1) ** $numberOfPeriods;
+        $tt2 = ($rate + 1) ** ($numberOfPeriods - 1);
+        $numerator = $futureValue + $tt1 * $presentValue + $payment * ($tt1 - 1) * ($rate * $type + 1) / $rate;
+        $denominator = $numberOfPeriods * $tt2 * $presentValue - $payment * ($tt1 - 1)
+            * ($rate * $type + 1) / ($rate * $rate) + $numberOfPeriods
+            * $payment * $tt2 * ($rate * $type + 1) / $rate + $payment * ($tt1 - 1) * $type / $rate;
         if ($denominator == 0) {
             return Functions::NAN();
         }
diff --git a/src/PhpSpreadsheet/Calculation/Financial/Coupons.php b/src/PhpSpreadsheet/Calculation/Financial/Coupons.php
index c24b31be..cdd54015 100644
--- a/src/PhpSpreadsheet/Calculation/Financial/Coupons.php
+++ b/src/PhpSpreadsheet/Calculation/Financial/Coupons.php
@@ -291,7 +291,11 @@ class Coupons
             return $e->getMessage();
         }
 
-        $yearsBetweenSettlementAndMaturity = DateTimeExcel\YearFrac::funcYearFrac($settlement, $maturity, 0);
+        $yearsBetweenSettlementAndMaturity = DateTimeExcel\YearFrac::funcYearFrac(
+            $settlement,
+            $maturity,
+            Helpers::DAYS_PER_YEAR_NASD
+        );
 
         return (int) ceil($yearsBetweenSettlementAndMaturity * $frequency);
     }
diff --git a/src/PhpSpreadsheet/Calculation/Financial/Securities/Constants.php b/src/PhpSpreadsheet/Calculation/Financial/Securities/Constants.php
index ba9d2389..f7bc2731 100644
--- a/src/PhpSpreadsheet/Calculation/Financial/Securities/Constants.php
+++ b/src/PhpSpreadsheet/Calculation/Financial/Securities/Constants.php
@@ -7,4 +7,7 @@ class Constants
     public const FREQUENCY_ANNUAL = 1;
     public const FREQUENCY_SEMI_ANNUAL = 2;
     public const FREQUENCY_QUARTERLY = 4;
+
+    public const END_OF_PERIOD = 0;
+    public const BEGINNING_OF_PERIOD = 1;
 }