From the book "The Meeker Family of Early New Jersey" the author Leroy J Meeker wrote "Unless the datings are all wrong, the first two children of Timothy, Joseph (c1737) carrying his grandfather Piersons's name, and Sarah (1/21/1738) whose birth-date is given as the day her mother died, were both the children of Sarah Pierson."
So although his date doesn't match the tombstone, the author thinks Sarah Pierson died in childbirth. I have no idea where 1/21/1738 was given, but it could be possible the tombstone is a year and eleven days off.