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Why do you use "monkey year, horse month" to indicate that you don't know when it will be?

The so-called monkey, horse and month refers to a long time in the future, which means you don't know when it will be, because according to the ancient time, it will take decades for the monkey, horse and month to get together, and heavenly stems and earthly branches is a 6-year cycle, so you will know how long it will take by yourself, so you will use this word to describe it.