What's best way to find closest values of datetime in pandas?

Question

import pandas as pd
import numpy as np
date1 = [\'2020-01-01\', \'2019-01-01\', \'2018-01-01\', \'2017-01-01\']
date2 = [\'2020-03-01\', \'2021-01-01\', \'2018-01-02\']