add the DAYS() function

src/PhpSpreadsheet/Calculation/Calculation.php

@@ -590,6 +590,11 @@ class Calculation
             'functionCall' => [DateTime::class, 'DAYOFMONTH'],
             'argumentCount' => '1',
         ],
+        'DAYS' => [
+            'category' => Category::CATEGORY_DATE_AND_TIME,
+            'functionCall' => [DateTime::class, 'DAYS'],
+            'argumentCount' => '2',
+        ],
         'DAYS360' => [
             'category' => Category::CATEGORY_DATE_AND_TIME,
             'functionCall' => [DateTime::class, 'DAYS360'],

src/PhpSpreadsheet/Calculation/DateTime.php

@@ -764,6 +764,49 @@ public static function DATEDIF($startDate = 0, $endDate = 0, $unit = 'D')
         return $retVal;
     }
 
+    /**
+     * DAYS.
+     *
+     * Returns the number of days between two dates
+     *
+     * Excel Function:
+     *        DAYS(endDate, startDate)
+     *
+     * @category Date/Time Functions
+     *
+     * @param mixed $endDate Excel date serial value (float), PHP date timestamp (integer),
+     *                                        PHP DateTime object, or a standard date string
+     * @param mixed $startDate Excel date serial value (float), PHP date timestamp (integer),
+     *                                        PHP DateTime object, or a standard date string
+     *
+     * @return int|string Number of days between start date and end date or an error
+     */
+    public static function DAYS($endDate = 0, $startDate = 0)
+    {
+        $startDate = Functions::flattenSingleValue($startDate);
+        $endDate = Functions::flattenSingleValue($endDate);
+
+        if (is_string($startDate = self::getDateValue($startDate))) {
+            return Functions::VALUE();
+        }
+        if (is_string($endDate = self::getDateValue($endDate))) {
+            return Functions::VALUE();
+        }
+
+        // Execute function
+        $PHPStartDateObject = Date::excelToDateTimeObject($startDate);
+        $PHPEndDateObject = Date::excelToDateTimeObject($endDate);
+
+        $diff = $PHPStartDateObject->diff($PHPEndDateObject);
+        $days = $diff->days;
+
+        if ($diff->invert) {
+            $days = -$days;
+        }
+
+        return $days;
+    }
+
     /**
      * DAYS360.
      *

src/PhpSpreadsheet/Calculation/functionlist.txt

@@ -76,6 +76,7 @@ DATEDIF
 DATEVALUE
 DAVERAGE
 DAY
+DAYS
 DAYS360
 DB
 DCOUNT

tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php

@@ -436,6 +436,22 @@ public function providerDATEDIF()
         return require 'data/Calculation/DateTime/DATEDIF.php';
     }
 
+    /**
+     * @dataProvider providerDAYS
+     *
+     * @param mixed $expectedResult
+     */
+    public function testDAYS($expectedResult, ...$args)
+    {
+        $result = DateTime::DAYS(...$args);
+        self::assertEquals($expectedResult, $result, null, 1E-8);
+    }
+
+    public function providerDAYS()
+    {
+        return require 'data/Calculation/DateTime/DAYS.php';
+    }
+
     /**
      * @dataProvider providerDAYS360
      *

tests/data/Calculation/DateTime/DAYS.php

@@ -0,0 +1,89 @@
+<?php
+
+return [
+    [
+        '#VALUE!',
+        '2007-1-10',
+        'ABC',
+    ],
+    [
+        '#VALUE!',
+        'DEF',
+        '2007-1-1',
+    ],
+    [
+        9,
+        '2007-1-10',
+        '2007-1-1',
+    ],
+    [
+        364,
+        '2007-12-31',
+        '2007-1-1',
+    ],
+    [
+        547,
+        '2008-7-1',
+        '2007-1-1',
+    ],
+    [
+        30,
+        '2007-1-31',
+        '2007-1-1',
+    ],
+    [
+        31,
+        '2007-2-1',
+        '2007-1-1',
+    ],
+    [
+        58,
+        '2007-2-28',
+        '2007-1-1',
+    ],
+    [
+        1,
+        '2007-2-1',
+        '2007-1-31',
+    ],
+    [
+        29,
+        '2007-3-1',
+        '2007-1-31',
+    ],
+    [
+        59,
+        '2007-3-31',
+        '2007-1-31',
+    ],
+    [
+        244,
+        '2008-9-1',
+        '2008-1-1',
+    ],
+    [
+        425,
+        '2008-4-1',
+        '2007-2-1',
+    ],
+    [
+        17358,
+        '2008-6-28',
+        '1960-12-19',
+    ],
+    [
+        9335,
+        '2008-6-28',
+        '1982-12-7',
+    ],
+    [
+        32,
+        '2000-3-31',
+        '2000-2-28',
+    ],
+    [
+        31,
+        '2000-3-31',
+        '2000-2-29',
+    ],
+];