Why did the ancients use heavenly stems and earthly branches to record the years and months?

At first, our ancestors only used the sky to record the days, because the days of a month were carried by the days; Remember the months with earthly branches, because ten months in a year exactly match ten earthly branches. However, not long after, people think that there will still be three days in a month when only the sky is dry to record the days. Therefore, a heavenly stem and a earthly branch are used to record dates in turn, such as Shangshu. Gu Ming says, "Only April is full of life. Kings don't make choices. Jia Zi, Wang Nai Tao Zanshui, was crowned with clothes. According to the records, this means that in early April, Wang felt very uncomfortable. On Jiazi Day, Wang Cai washed her hair and face. The servant dressed the king, and Wang Yi sat on the jade table. Later, the method of recording the day was gradually introduced to record the year, month and time.