Gardner: 'Ann, w. Edward Coffin (s. Peter and Abigail), "No chn.," d. John and Priscilla (Grafton), 30th, 12 mo. 1667 [date written above 17th, 5 mo. 1664, queried twin of Benjamin] [in Salem], P.R.38.'
↑ The date given in Salem's Vital Records - 30: 12m: 1667 - would be 30 Feb 1667/68, which doesn't exist. It is also too close to the date of Nathaniel's birth (24 Sep 1668).
The Nantucket record was created from a "private record" after Ann was married and probably after 1752 when May became the 5th month, since its note about 17th, 5 mo. 1664 is clearly meant to be Benjamin’s 17 May 1664 birth date. The question about whether she was Benjamin’s twin could be an attempt to resolve the issue of Ann’s clearly incorrect birth date, but it seems more likely that she was born about 1666 (although not 30 Feb).