In some cases a serial number may leave you with a fuzzy span of several years, and in others you will know which number your guitar was within a batch during a specific month and year. While the situation isn't quite as bad as say, Gibson or Guild, this guide should be viewed as the best available consensus, not gospel.
Like the other Giants of Original American Guitars (save Martin), Gretsch had its share of corporate hand-offs and resulting serial number inconsistencies over the years.