Until you check a parish register or, after Sept 1837, the civil registration (or both, because people early on would not always register a birth), you can't be positive of a birth/baptism date.
In the case of death, the information is only as good as the person who supplied it.
All of which points to why one needs to find parish registers (which will still have births/baptisms even after civil reg came into being), censuses, marriages, and deaths documents (the last two also being in parish registers).