"Earliest Baptismal Records of the Church of Harlingen (Reformed Dutch) of New Jersey, 1727-1734," was published by William Jonas Skillman in the New York Genealogical and Biographical Record, volume 40 (1909). The baptism register, which covers the years 1727 to 1734, is preceded by an introduction on the churches of the Raritan Valley in New Jersey which most researchers should find informative. At the time these baptisms took place, the church was not known as Harlingen, but rather as Sourland, or op de Millstone (the church beyond the millstone). The name, Harlingen, was adopted in 1766. There are apparently breaks in the records after 1734, and they appear to be sporadic until 1744. Publication of the baptisms was picked up by the Genealogical Magazine of New Jersey, volume 17 (1942), beginning with the year 1737.
Early Baptisms, Reformed Dutch Church, Harlingen, N. J., NYGBR v.40 (1909)