Princess Antoinette of Monaco, Baroness of Massy (Antoinette Louise Alberte Suzanne Grimaldi; 28 December 1920 – 18 March 2011) was a member of the princely family of Monaco and the elder sister of Prince Rainier III and aunt of Albert II, Prince of Monaco. Her parents were Count Pierre de Polignac and Princess Charlotte, Duchess of Valentinois.
Princess Charlotte, Duchess of Valentinois was born on 28 December 1890.
It's never stated in the text when she was born, so we don't know for sure. It's difficult because it's a similar birthday with a different name compared to the one in the text.