What dynasty is it outside the mountain?

There are mountains beyond the mountains, and one day outside the mountains belongs to the Tang Dynasty. Judging from the relevant public information, beyond a certain realm, it is an endless realm. Often used to express that people's horizons are limited by objective conditions, and the field of knowledge needs to be continuously expanded. This poem appeared in the prosperous time of Kaiyuan in Xuanzong period of Tang Dynasty, calling on people to make continuous progress and forge ahead bravely.