I do not understand what your issue actually is, but,
1) 'YYYY-MM-DD' is the preferred default date format,
2) Excel date cells can be formatted using the ExcelFmt function in the Spreadsheet::ParseExcel::Utility library, and
3) The Oracle default date/timestamp formats can be set like so (notice I set it to my preferred format, and I do this for every connection in my connect wrapper library):

      $dbh->do("alter session set nls_date_format='YYYY-MM-DD HH24:MI:
+SS'");
      $dbh->do("alter session set nls_timestamp_format='YYYY-MM-DD HH2
+4:MI:SS.FF'");
      $dbh->do("alter session set nls_timestamp_tz_format='YYYY-MM-DD 
+HH24:MI:SS.FF'");
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Update: Also note that even though the Oracle format includes a time component, it will still accept a string without the time in the format 'YYYY-MM-DD'.