August 1998-July 1999 is the date of the index, not the date of the article. The date of the index usually indicates when that particular volume was published. Because publishing a print volume is a lengthy process, the articles that are listed within an index are sometimes a little older than the date on the index.
That's why you can look in a 1999 periodical index and find articles that were published in 1998 (or even earlier sometimes).
Go back and look at the citation again and choose another option.