According to the catalogue description, the letter is dated 28 April. The year is conjectured by the relationship between this letter and an exchange of letters between CD and Claparède in 1862; the only extant letters between them are the letter to Edouard Claparède, [c. 16 April 1862], and the letter from Edouard Claparède, 6 September 1862 (Correspondence vol. 10).