In addition to what shadowsong points out, the date string that you pass to write_date_time() must be in ISO8601 format ( 'yyyy-mm-ddT' or 'yyyy-mm-ddThh:mm:ss.sss' ) 
my $format = $workbook->add_format(num_format => 'mm/dd/yy');
$worksheet->write_date_time(0, 1, '2004-05-13', $format); # wrong
$worksheet->write_date_time(1, 1, '2004-05-13T', $format); # right

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Results in: 
Row, Col    = (0, 1)
Value       = 2004-05-13

Row, Col    = (1, 1)
Value       = 05/13/04

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In reply to Re: Creating Excel file using Perl by tangent
in thread Creating Excel file using Perl by Rahul Saxena

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