In September 1828 Chopin, while still a student, visited Berlin with a family friend, zoologist Feliks Jarocki, enjoying operas directed by Gaspare Spontini and attending concerts by Carl Friedrich Zelter, Felix Mendelssohn and other celebrities. On an 1829 return trip to Berlin, he was a guest of Prince Antoni Radziwiłł, governor of the Grand Duchy of Posen—himself an accomplished composer and aspiring cellist. For the prince and his pianist daughter Wanda, he composed his Introduction and Polonaise brillante in C major for cello and piano, Op. 3.

Answer this question, if possible (if impossible, reply "unanswerable"): When did Chopin visit Berlin?
September 1828