SPOONER, Seth, s. Samuel Jr. and Deborah, [born] Jan. 31, 1755-6 [sic, see following comment.]
[Note: Clearly either parents or date is wrong. Suspect the date because double dating not used for 1755 or 1756. There was a Samuel Jr. and Susanna having children in the 1750s, including children born in Jan. 1753, May 1754, and Jul 1756. So it would be hard to argue this child could be theirs, with a misrecorded mother. Further, we know Samuel Jr. and Deborah did have a son Seth, who was deceased when the father wrote his will in 1777. So it seems the parents are most likely correct, and that the date must be transcribed wrong.
Source:Spooner, Thomas. Records of William Spooner of Plymouth, Mass. and His Descendants, p. 52, shows for Seth, the son of Samuel and Deborah, a birth date of 31 Aug 1735 and death date of 11 Dec 1768. There is a death record in Nantucket of the "h. Dinah" on the 11th of the 11th month in 1768 for Seth Spooner "from Dartmouth". According to p. 92 Seth married Dinah Gardner, and since the Nantucket record was contributed by William Folger, more research is necessary to resolve the one-month discrepancy between these two secondary sources. But this is enough time for a man born in 1735/36 to have the children mentioned in his father's will in 1777, and importantly for this discussion, suggests the 1755-6 is not a death date mistaken for a birth.
Resolution of the birth date is found by inspecting the online records from Dartmouth. Seth is recorded in a family register with all of Samuel's children p. 206. The date is somewhat ambiguous due to a stray mark, but it most appears to be 1735/6 and the inclusion in this family register, and the double-dating, by context, confirm that this must be the correct interpretation. The two other records found online (Dartmouth town copy and family register in New Bedford), both use this same interpretation. The August date found in the Spooner genealogy cannot be justified by anything found online, though one could imagine there could be an even older copy of the records, or family Bible, where "Jan'y' was misread as Aug?]