while ( <DATA> )
{
   s!\b((\d{4}[-/]\d{2}[-/]\d{2}\b)|(\d{2}[-/]\d{2}[-/]\d{4}))\b!<date>$1</date>!g;
   print;
}


__DATA__
2000-04-23 was a great day, but 04-24-1999 is much better.
2000/04/23 I would like to match those 3 formats for now.
2000-85-85 is not a real date.