I'm not sure. I seem to recall that the language implied that Dumbledore co-created it with Flamel, which would mean 20th century,

The wording in book one which you may be thinking of just says that Dumbledore and Flamel did alchemical work together. Flamel's construction of the philosopher's stone is not mentioned until much later when Hermione finds the reference and there it just says that Flamel is the only person known to have made one.