datetime error while converting string to datetime in a If statement

I have read this question asked several times but I just can't get rid of the datetime attribute error. Managed to import a csv file and assigned the latest datetime to "LastImportDate". Printed out the dates without issue but encountered error when I added in the if statement to compare "LastImportDate" and "Yesterday". Can advise me what has gone wrong on my script? Thanks in advance.

=====

import pandas as pd
import numpy as np
from datetime import datetime, timedelta

LastTankGaugeBook = pd.read_csv('Final_Tank_Gauge_Book.csv')
LastTankGaugeBook = LastTankGaugeBook.set_index(['Date_Time'])
LastTankGaugeBook = LastTankGaugeBook.sort_index()

LastImportDate = LastTankGaugeBook.index.max()
Today = datetime.today()
Yesterday = Today - timedelta(days=1)
print('Last import date: ', LastImportDate)
print('Today date: ', Today)
print('Yesterday date: ', Yesterday)

def date_stamp(date):
    return datetime.datetime.strptime(date,'%Y/%m/%d')

if date_stamp(LastImportDate) >= date_stamp(Yesterday):
    print('No import operation')
else:
    print('Start import operation')

=======

Output:
Last import date:  2019-01-11 23:00:00
Today date:  2020-06-15 06:28:49.492720
Yesterday date:  2020-06-14 06:28:49.492720
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-131b8da9ab87> in <module>
     42     return datetime.datetime.strptime(date,'%Y/%m/%d')
     43
---> 44 if date_stamp(LastImportDate) >= date_stamp(Yesterday):
     45     print('No import operation')
     46 else:

<ipython-input-8-131b8da9ab87> in unix_stamp(date)
     40
     41 def date_stamp(date):
---> 42     return datetime.datetime.strptime(date,'%Y/%m/%d')
     43
     44 if date_stamp(LastImportDate) >= date_stamp(Yesterday):

AttributeError: type object 'datetime.datetime' has no attribute 'datetime'