I doubt there’s any source for her birth date.
The more reliable study has:
https://cybergata.com/roots/9637.htm
Called in NGR. lady of the manors of Horton near Canterbury and Bewsfield (otherwise Whitfield) near Dover. These appear in her Inq.p.m. Hasted says she was daughter of Guncelin (Joscelin) de Badlesmere, who gave these manors with her in free marriage to John de Northwode. No deed of such gift has been found, but John was said to hold 10 marks rents in Harrietsham of the gift of Guncelin. Her age would make it likely that Joan was sister, not daughter, of Guncelin.
~Cokayne's Complete Peerage, 2nd Edition, Vol. IX, pp. 754-755
The more modern encyclopedic citation:
https://en.m.wikipedia.org/wiki/John_Northwood,_1st_Baron_Northwoo
About 1275 he married Joan Badlesmere, daughter of Sir Guncelin Badlesmere, and they had six sons. She died on 2 June 1319, a week after her husband, and is also commemorated by a brass at Minster-in-Sheppey.
5. C. L. Kingsford; Andrew Ayton (23 September 2004). "Northwood, John, first Lord Northwood (1254–1319)". Oxford Dictionary of National Biography. Retrieved 10 September 2023
https://en.m.wikipedia.org/wiki/Guncelin_Badlesmere
From a family of minor gentry in the village of Badlesmere, who had served as knights and judges, he was born before 1243 as the son of Batholomew Badlesmere.[3]
[3] Edward Hasted (1798). "'Parishes: Badlesmere'". The History and Topographical Survey of the County of Kent. Vol. 6. Canterbury. pp. 467–481. Retrieved 3 September 2023.
