I have just come back from Windsor Library where they have a remarkable collection of local history documents and microfiche of church records etc.
The Journal and Proceedings of the Royal Australian Historical Society, Vol. XXXII Part II, 1946 contains an apparently well researched article by Cecil W. Salier, ‘The Life and Writings of Charles Harpur’, which had been presented to the Society’s ‘Charles Harpur Commemoration Evening’ on the 1st November 1945.
On the opening page it says;
“Charles, the third of the seven children of Sarah and Joseph Harpur, was baptized at St Matthew’s Church, Windsor, New South Wales, on May 23, 1813, the date of birth being then given as January 23, 1813. Harpur however, himself believed he was born ‘about 1817.’”
This may have been because, in another article from a local paper, it stated his parents were not married until June 1814.
This is confirmed in the Hawkesbury Pioneer Register;
Husband’s name: CHARLES CHIDLEY HARPUR
It goes on to say he was born at the school residence in Windsor (his father was the school master) and that he married MARY ANN DOYLE (born 18/10/1820, Windsor, Died 1899 and buried at Eurabadalla) etc etc.
So my assumption is that when the poems were published, Harpur’s birthdate was stated as being 1817 in accordance with what Harpur had said all his life and so also avoiding any stigma associated with being illegitimate.