Why not go golang style, unix timestamp as nanoseconds, in signed int64. Maybe you can't cover millions of years with nanosecond precision, do you really need it?
With that precision and size, you can only cover the years from 1678 to 2262, which strongly limits your ability to represent historical dates and times.
If you're representing dates back into the 1600s you need to keep in mind that calendar maths and things like "was this year a leap year" become more complicated. The Gregorian calendar was introduced in the 1500s but worldwide adoption took a long time - for example, the UK didn't adopt it until the 1700s. So you've got more than a century where just having "a date" isn't really sufficient information to know when something happened, you'll need to also know what calendar system that date is in.
Overall, this means if you're representing historical dates I would question whether a seconds-since-epoch timestamp representation is what you want at all, regardless of range and precision.
Edit: yes, you can kinda handle this as part of handling timezones, but still, it's complicated enough that you may want to retain more or different information if you're displaying or letting users enter historical dates.
With nanosecond precision? Just decide what you want to do beforehand, i bet even datetime don't make much sense for that time period, bare date would suffice. also, you'll likely need location, calendar system etc since real dates were not that standardized back then
