Things get ugly if you go back far enough that you need to account for jurisdictions which no longer exist switching between calendars at different times from one another. I don't know how well Unix timestamps will fare for dates prior to approximately the 1600s.
At least you won't need to worry about figuring out historical leap seconds.
> Things get ugly if you go back far enough that you need to account for jurisdictions which no longer exist switching between calendars at different times from one another.
I think that would be a 'timezone' conversion you do at display time. Internally, it's still stored as a unix timestamp.