Date formats in excel can be tricky. See Excel::Writer::XLSX::Utility

#!perl
use strict;
use Excel::Writer::XLSX;
use Excel::Writer::XLSX::Utility;
use Time::Piece;

my $wb = Excel::Writer::XLSX->new( 'dates.xlsx' );
my $ws = $wb->add_worksheet(); 
my $fmt = $wb->add_format();
$fmt->set_num_format( 'dddd mmm dd, yyyy' ); 

my $date = '3/15/16, 00, Mar, 2016';
my $t = Time::Piece->strptime($date, '%D, %M, %b, %Y') ;
my $xldate = xl_date_list($t->year, $t->mon, $t->mday);   

$ws->write( 0, 0, $xldate, $fmt );
$wb->close();
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