diff --git a/src/PhpSpreadsheet/Style/NumberFormat.php b/src/PhpSpreadsheet/Style/NumberFormat.php
index ac64b20e..5f23edcc 100644
--- a/src/PhpSpreadsheet/Style/NumberFormat.php
+++ b/src/PhpSpreadsheet/Style/NumberFormat.php
@@ -24,7 +24,7 @@ class NumberFormat extends Supervisor
 
     const FORMAT_DATE_YYYYMMDD2 = 'yyyy-mm-dd';
     const FORMAT_DATE_YYYYMMDD = 'yy-mm-dd';
-    const FORMAT_DATE_DDMMYYYY = 'dd/mm/yy';
+    const FORMAT_DATE_DDMMYYYY = 'dd/mm/yyyy';
     const FORMAT_DATE_DMYSLASH = 'd/m/yy';
     const FORMAT_DATE_DMYMINUS = 'd-m-yy';
     const FORMAT_DATE_DMMINUS = 'd-m';