Bad books on writing tell you to "WRITE WHAT YOU KNOW", a solemn and totally false adage that is the reason there exist so many mediocre novels about English professors contemplating adultery.

Don't 'write what you know.' Make up something new!

There's no such thing as writing about the future. The future hasn't happened yet.

The worst advice a young writer can get is "Write what you know." Imagination is more important than experience.