Orson Pratt quotes Lyman Johnson as saying that Smith knew about plural marriage in 1831.
Orson Pratt, “Report of Elders Orson Pratt and Joseph F. Smith,” The Latter-day Saints' Millennial Star, vol. 40, no. 50, December 16, 1878, 788
"Lyman Johnson, who was very familiar with Joseph at this early date, Joseph living at his father's house, and who was also very intimate with me, we having travelled on several missions together, told me himself that Joseph had made known to him as early as 1831, that plural marriage was a correct principle. Joseph declared to Lyman that God had revealed it to him, but that the time had not come to teach or practice it in the Church, but that the time had not come to teach or practice it in the Church, but that the time would come."