This reminds me that centuries such as "the third century BC" are even harder to translate into date ranges. That one's 201 BC to 300 BC, inclusive, backward. Or you might see "the last quarter of the second millennium BC", which means minus 2000 to about minus 1750. [Edit: no it doesn't.]
In fact archeologists have adapted to writing "CE" and "BCE" these days, but despite that flexibility I've never seen somebody write a date range like "the 1200s BCE". But they should.
Some people have proposed resetting year 1 to 10,000 years earlier. The current year would be 12024. This way you can have pretty much all of recoded human history in positive dates, while still remaining mostly compatible with the current system. It would certainly be convenient, but I don't expect significant uptick any time soon.
For earlier dates "n years ago" is usually easier, e.g. "The first humans migrated to Australia approximately 50,000 years ago".
> you might see "the last quarter of the second millennium BC", which means minus 2000 to about minus 1750.
From comparing some online answers (see links), I'd conclude that even though the numbers are ordered backward, "first"/"last"/"early"/"late" would more commonly be understood to reference the years' relative position in a timeline. That is, "2000 to about minus 1750" would be the first quarter of the second millennium BC.
In fact archeologists have adapted to writing "CE" and "BCE" these days, but despite that flexibility I've never seen somebody write a date range like "the 1200s BCE". But they should.