The title of Li Zicheng is Dashun, because when Zhu Yuanzhang established the Ming Dynasty, he once asked his then minister Liu Bowen to divine and calculate how long the title of the Ming Dynasty was. Liu Bowen only said after divination that once he met Shun, the Ming Dynasty would perish. And this sentence has been handed down.
Later, Li Zicheng's strategist, a person who likes divination very much, naturally studied what Liu Bowen calculated in those years, and the word Shun also means "Tianshun Ming". As an important counselor under Li Zicheng, the strategist naturally suggested that Li Zicheng set the title as Dashun. In fact, this is also the reason why those ancient rulers deliberately pulled some untrue historical things to talk nonsense in order to consolidate their rule.
It is also to show that the throne he inherited is orthodox. In later Zhang, Dashun was designated as the national title. This is because Li Zicheng was defeated and fled by the Qing army, and Zhang just gathered Li Zicheng's original troops. In order to show that he got Li Zicheng's troops, he should get something, so Zhang, that is, set his title as Dashun. In fact, whether Li Zicheng or Zhang, they use Dashun as the country name or year number, just to show that they are the orthodox heirs in the world.
In the final analysis, this is just a means to confuse people. However, it is said that there was a contradiction between Zhang and Li Zicheng, so Zhang mocked Li Zicheng in the name of Dashun after seeing Li Zicheng's army rout.