John is too young to be the father of Joseph (b. 1794) — see Children below — unless one of their birthyears is wrong, and I'll bet it's John's because men rarely married women older than themselves.  The birth and death dates for John and Julianna appear to be from tombstones, which can be weathered and misread.  Also, the person ordering the stone may be mistaken.  Women, on average, marry at age 19.  If Joseph is, indeed, their first child and his birthyear is correct, then Julianna's birthyear of 1775 is reasonable.  Men, on average, marry at age 23 and are four years older than their wives.  That would make a birthyear of 1770/71 for John much more reasonable.

1.  Mea culpa... I forgot to note my source, but it was surely some site on the web, meaning everything here is from a secondary source.