Date format compatible with both LibreOffice and Excel

Date formats used invalid single `y`, instead of double `yy`.
That was was implicitly fixed and displayed correctly by Excel. But
LibreOffice does not implicitly fix it and instead display the literal `y`
instead of th year value.

See also: https://support.office.com/en-us/article/Format-numbers-as-dates-or-times-418bd3fe-0577-47c8-8caa-b4d30c528309

Fixes #298

src/PhpSpreadsheet/Style/NumberFormat.php

@@ -25,16 +25,16 @@ 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_DMYSLASH = 'd/m/y';
-    const FORMAT_DATE_DMYMINUS = 'd-m-y';
+    const FORMAT_DATE_DMYSLASH = 'd/m/yy';
+    const FORMAT_DATE_DMYMINUS = 'd-m-yy';
     const FORMAT_DATE_DMMINUS = 'd-m';
-    const FORMAT_DATE_MYMINUS = 'm-y';
+    const FORMAT_DATE_MYMINUS = 'm-yy';
     const FORMAT_DATE_XLSX14 = 'mm-dd-yy';
     const FORMAT_DATE_XLSX15 = 'd-mmm-yy';
     const FORMAT_DATE_XLSX16 = 'd-mmm';
     const FORMAT_DATE_XLSX17 = 'mmm-yy';
     const FORMAT_DATE_XLSX22 = 'm/d/yy h:mm';
-    const FORMAT_DATE_DATETIME = 'd/m/y h:mm';
+    const FORMAT_DATE_DATETIME = 'd/m/yy h:mm';
     const FORMAT_DATE_TIME1 = 'h:mm AM/PM';
     const FORMAT_DATE_TIME2 = 'h:mm:ss AM/PM';
     const FORMAT_DATE_TIME3 = 'h:mm';