While a date range is preferable, you have to use what you have got. eg. The only thing I know about the father of my great aunt, who was born illegitimate, is her father's name and the date she was born (ie. I know he was alive about 9 months earlier, but I have no idea how old he was, etc). Despite quite a few hours spent trying to find out more aboute him the only thing I can say with certaintly about Charles Ferris is: "fl. 1934".