Can't say which is correct, but Paul Worth (follow link to earlier discussion) says the poem was written in 1864, before her death, but Fawcett-Johnston in the notes above says it was written the year she died, 1866. Worth's story makes more sense to me, not to mention the short time frame involved in her death, Johnson's writing the poem, giving it to Butterfield, Butterfield's composing the music, and publishing it, all by May of 66.