In that case try the write_date_time solution Tux suggested here

#!/usr/bin/perl 
use strict;
use Spreadsheet::WriteExcel;
use DBI;
my $dbh = get_dbh(); # connect as req

my $workbook = Spreadsheet::WriteExcel->new( 'perl.xls' );
my $worksheet = $workbook->add_worksheet();

# set column format
my $datecol = 19;
my $date_format = $workbook->add_format(num_format=>'m/d/yyyy h:mm');
$worksheet->set_column($datecol,$datecol,15,$date_format);#width 15

my $sql = "SELECT * FROM test21";
my $sth = $dbh->prepare($sql);
$sth->execute;

my $row2 = 1;
while (my $aref = $sth->fetchrow_arrayref){
 
  # convert to ISO8601 yyyy-mm-ddThh:mm:ss.ss
  my $date = $aref->[$datecol];
  $date =~ s/ /T/;
  print "$date\n";
  $aref->[$datecol] = '';

  $worksheet->write_row($row2,0,$aref);
  $worksheet->write_date_time($row2,$datecol,$date);
  ++$row2;
  
}   
$workbook->close;
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