It sounds like the death date is okay, but the birth date is in question. I'd suggest that you look for additional birth date sources. You know, people may be pretty upset when they provide information for the death certificate, and sometimes don't even know the correct date (hence the wrong birth year on my father's death certificate, Mom was wrong).
For sources, try census records, marriage records, military records, in the US Social Security records (for which you had to show proof of birth). Also check the birth dates of siblings against the 9 month apart sanity check and the marriage date of the parents (although that could account for a year's difference in the birth date). You could also search for obituaries.