Vague dates

I'm sure I posted this message earlier today, so let me know (and
apologies) if it shows up somewhere...

I'm creating a table of antique objects, including their year of
manufacture. In some cases the year will be precisely known - '1855'
or '1930', say. In other instances it will be vaguer - 'circa 1850' ,
'1835 - 1860' or 'mid-19th century'. Just wondering if anyone had any
thoughts on how best to represent this in the table. One idea I had
was simply to provide, in each case, a mid-range value in one column
and a margin of error or 'tolerance' in another - but I'm open to
suggestions.

Any thoughts - or, better yet, a definitive answer, appreciated.