In which period did Yan Zhenqing's works refer to "Old Books at the Meeting"?

At the meeting, people and books always mentioned Yan Zhenqing's later works. By inquiring about China's calligraphy and official website, we know that Yan Zhenqing's works are all familiar in his later years at the General Meeting. Personal style is strong, and his works are more exquisite and free than before. Vigorous and strong. "At the meeting, people and books are old." Sun, a calligrapher in the Tang Dynasty, wrote this book. It means that it takes a person's life to have a deep understanding and real grasp of calligraphy.