We have a dataframe like
我们有一个数据帧
Out[90]:
customer_id created_at
0 11492288 2017-03-15 10:20:18.280437
1 8953727 2017-03-16 12:51:00.145629
2 11492288 2017-03-15 10:20:18.284974
3 11473213 2017-03-09 14:15:22.712369
4 9526296 2017-03-14 18:56:04.665410
5 9526296 2017-03-14 18:56:04.662082
Out[90]: 
