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 bash:
    date -d"1899-12-30 +40254 days"
...
 MySQL:
    DATE_ADD('1899-12-30', interval 40254 day)

sub get_excel_date
{
...
    my @x = localtime($date*24*3600 - 2209143600);
    return sprintf("%04d-%02d-%02d", $x[5]+1900, $x[4]+1, $x[3]);
}