The only problem here is to understand which date format was used.
Perhaps your location will help here, since in the USA the MM/DD/YYYY format is more often used and in the European part DD/MM/YYYY

I think that if you have any records of your father or messages, correspondence where he used some date assignments or something related to it, then you can at least understand what format he has and then parse all possible dates starting from 1970, as indicated by @NotAtether, I think it will make your search much easier.