Forum:Harry Potter years

Hi,

I was just wondering if anyone knew where year 1998 for the final Battle of Hogwarts came from. It seems to be the agreed year, however on lookinat the JKR's hand-written Weasely family tree (decending from Molly and Arthur), she's written Fred Weasley d. 1997. As Fred died in the Battle of Hopgwarts, shouldn't the year of the battel be 1997?