我就这样解决了，而且成功了。有人可能对解决办法感到疑惑。

import pandas as pd
from xlwt import Workbook
from datetime import datetime

df = pd.read_excel('yourexcell file.xlsx')
date=[]

for i in df.index:
    start_date = df['date'][i] #date column name in excel file 
    start_time=df['time'][i] #time column name in excel file

    date_time = datetime.strptime(start_date, "%d.%m.%Y")

    comb=datetime.combine(date_time,start_time)

    date.append(comb)

# dateandtime combined and ready.

wb = Workbook()
sheet = wb.add_sheet('Sheet 1')

for k in range(len(date)):
    sheet.write(k,0,date[k])
wb.save('datetime_generated'+".CSV")
print("csv file saved.")

这里是：输出2018-05-27 00:00:00