Allow start and end date of DATEDIF function to be the same

src/PhpSpreadsheet/Calculation/DateTime.php

@@ -664,7 +664,7 @@ class DateTime
         }
 
         // Validate parameters
-        if ($startDate >= $endDate) {
+        if ($startDate > $endDate) {
             return Functions::NAN();
         }
 

tests/data/Calculation/DateTime/DATEDIF.php

@@ -607,4 +607,10 @@ return [
         'Y',
         50,
     ],
+    [
+        '1982-12-07',
+        '1982-12-7',
+        'D',
+        0,
+    ],
 ];