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George Fordyce Wilson, illegitimate son of Jean Wilson, was born in Cullen on 13 January 1864, exactly seven months to the day after Peter Fordyce married Isabella McEwen.
I presume you actually have the 1911 census, because the 1901 doesn't say how long a couple have been married?
In the 1901 census George W Fordyce and his wife Helen have four children, the eldest of whom was born in the United States and the rest in Marnoch. The birth certificates of the other three will tell you the date and place of the marriage.
FamilySearch has an index listing of a marriage of George Fordyce and Helen Beattie in Chicago on 25 September 1891.
In the 1871 and 1881 censuses, George Fordyce, born Cullen, is in the household of Robert Thomson, whose wife is Elizabeth Fordyce. In 1871 he is described as nephew.