A letter to Hallam Tennyson by Julia Margaret Cameron

As usual, in reading about Alfred Tennyson, I came across a letter written by Julia Margaret Cameron to the eldest son of Alfred Tennyson, Hallam Tennyson. He was only three years old at the time and I thought Mrs. Cameron's tone was very endearing. I wonder though, her letter discusses Hallam's birthday (which was August 11, 1852 and her letter is dated March 15 1856. Either she missed his birthday on August 11 1855 or she's preparing for his upcoming birthday August 11 1856? I just thought the date of her letter and his birthday being mentioned was quite interesting. In her letter, she references three of her children: her daughter Juley (Julia), her sons Charlie and Henry as well. Read the letter for yourself, Photograph of Hallam Tennyson, son of Alfred, Lord Tennyson, by Lewis Carroll (Charles Dodgson) 28 September 1857, Albumen print, (formerly owned by the Weld family) 15 March 1856 My darling little Hallam I have not forgo