William Bert (Burt's) birth date

I am just curious as to where the April 2, 1799 birth date is found. I have a handwritten list of birth dates and deaths, written in the 1890s by Sarah A. Simmons, daughter of William and Amanda Chipman Simmons where she lists "William B. Simmons the son of Edward and Celia Simmons born in Westmoreland in the state of New Hampshire, April 4, 1700. Died Aug 20, 1899."