Don Bradley dates the discovery of the Smith-Alger marriage between April-September 1836.
Don Bradley, "Mormon Polygamy Before Nauvoo? The Relationship of Joseph Smith and Fanny Alger," in The Persistence of Polygamy: Joseph Smith and the Origins of Mormon Polygamy, ed. Newell G. Bringhurst and Craig L. Foster (Independence, MI: John Whitmer Books, 2010), 23
Cowdery's 1837-1838 complaints of Smith's "dirty, nasty, filthy" relationship with Alger are also significant by their absence in early 1836, when Cowdery had no apparent trouble accepting Smith's purity and worthiness to dedicate the Kirtland temple on March 27, and to see Christ with him on April 3. This too suggests a later dating for the "outing" of the relationship, placing it probably between April 3, 1836, and September of the same year.