Flourish

We are told that a writer *flourished* at such and such a time. Is any definite notion attached to this word? When it is said of a century there is no difficulty; it means that the writer was born and died in that century. But when we are told that a writer flourished about the year 1328 (such limitation of florescence is not uncommon), what is then meant? What are we to understand he did in or about 1328?