I don't think the line should be set only in time, but also in contribution. Did the person contribute something which is now foundational? For example some thi

I don't think the line should be set only in time, but also in contribution. Did the person contribute something which is now foundational? For example some thing in the culture or technology or something that made a real difference to the future of the company? Then they should be given co-founder status nevertheless. It's been mentioned before, but making someone co-founder retroactively seems reasonable. I'd go as far as say that someone who are present from day 0, but does not manage to contribute in any lasting or meaningful way, may be unconsidered as co-founder.