Since JT was 10 years older than his wife suspect he may have knocked a few years off his age when he married but by 1939 the truth was known. People often get the year wrong but not their actual birthday. I go with the John whose certificate you have who was born the exact date 18/6/1876, the date he states he was born on 1939 Register. Plus, as already proven, the John Thomas Pierce born Flintshire in 1879 was son of a Daniel.
Annette