p 234 - 234 Deed of Joseph Bridger, Esq., for certain land (left by Dr. Robert Williamson to Joan, his wife, for life, and then to his eldest son Robert, which lands were found to escheat and were then granted in 1678 to said Bridger) to said Joan, now the wife of Mr. Robert Burnett. 8 Aug. 1683. ----- [Proves that Joan was married to Robert Williamson and Robert Burnett.]
The author estimates Joan's birth year as 1638 (and her marriage year as 1652), with an explanation, although there is no reason she couldn't have been born a few years earlier.