Very often, before the advent of birth certificates (varies by country, but 1915 in the US), unrecorded birthdates were estimated after the event for census records, marriage licences, draft records, ect. Those dates do not even always agree with one another, but a geneologist needs to support claims by documentation.
Even when the dates are a little illogical, the geneologist does not change them so that others can use the information to find the records for varification of documentation. If all other information: locatlity, relationships, approximate dates, etc. agree, then one can resonably demonstrate that the record, though chronologically improbable, supports a link between family members.
