In ancient times, it didn't change until one year after the death of the great emperor. If the newly succeeded emperor has been in office for less than a year, how can he become an adult?

On July 21st (1620 August 18), the emperor died. On the first day of August (1620, August 28th), Zhu Changluo ascended the throne, with an unusual title. After Zhu Changluo succeeded to the throne, he faced the grim situation left by Emperor Wanli, such as the corruption of the imperial platform, economic recession, constant palace disputes, declining military strength, tense border, the rise of Jurchen, the broken government and the crumbling Daming Mountain. Zhu Changluo, who is weak, withdrawn, depressed and lustful, is unable to save the situation that is about to collapse. Emperor Taichang was emperor for only one month, and died on the first day of September (1September 26th, 620). He was called "the son of heaven in January" in history. Therefore, the calendar year is forty-eight years of Wanli, so October to July of the forty-eight years of Wanli (1620) is designated as forty-eight years, and the beginning of August to the end of the year is designated as the first year of Taichang, so the calendar year of Wanli is still forty-eight years, while the calendar year of Taichang is only four months. -Baidu Encyclopedia.

The following year is the first year of the apocalypse.