His attitude today is completely different from yesterday's.
Tips
history
From Zhang Jie's Song-dynasty 《岁寒堂诗话》: "文章古今迥然不同" — ancient and modern writing are worlds apart. 迥 originally means "far, distant," so the phrase literally says "far apart in kind."
usage
Stronger than 不一样 or 不同. Use when the contrast is genuinely stark. Common collocation: A 与 B 迥然不同 / A 和 B 迥然不同.