How did Zhuge Liang calculate by hand?

It's just an interpretation on TV. Ancient people paid special attention to the weather. Seemingly mysterious, it's actually just a way to calculate heavenly stems and earthly branches:

The cooperation between the ten-day trunk and the twelve-day branch generally requires a pen and a piece of paper, but the ancients rarely had the opportunity to use it when calculating numerology. So divide your fingers into three sections, just fix them for twelve days, and read the earthly branches according to your knuckles when matching them, so that you can calculate the heavenly stems and earthly branches of the year and month more quickly.