You can calculate the time according to the events and places involved in the poem. For example, Pipaxing, you can check the background of this sentence "Jiangzhou Sima's blue shirt" and find out when Bai Juyi was demoted to Jiangzhou Sima. Another example is Sauvignon Blanc, "Bianshui, Surabaya, flows to Guazhou ancient ferry, and Wu Shan is a little sad." Guazhou ferry in Zhenjiang today has been an important ferry connecting Yangzhou Zhenjiang since ancient times. Why did Bai Juyi go to that place? Wu Shan is a little sad. Zhenjiang is in the south and Yangzhou is in the north. Wu Shan refers to the scenery of Wu, and when he arrived in the east of Runzhou (Zhenjiang), he arrived in wuyue. That means Bai Juyi is going to Jiangnan Road, so why did he go to Jiangnan Road? Bai Juyi has another poem called "Recalling Jiangnan", and we know that there are Bai Causeway and Su Causeway in the West Lake of Hangzhou, among which Bai Causeway is the "White Sand Embankment in the Shade of Green Yang" (spring outing of Qiantang Lake) that the world thinks Bai Juyi built. So you can find out when Bai Juyi went to Hangzhou to make a secretariat.
other poems and so on ...
Do you understand?