import datetime as dt
# current time
cur_time = dt.datetime.today()
# one day
pre_time = dt.date(2016, 5, 20) # eg: 2016.5.20
delta = cur_time - pre_time
# if you want to get discrepancy in days
print delta.days
# if you want to get discrepancy in hours
print delta.hours
# and so on

cur_time = dt.now()
# previous n days
pre_time = dt.timedelta(days=n)

cur_time = dt.now()
# get day of current time
cur_day = cur_time.replace(hour=0, minute=0, second=0, mircrosecond=0)

cur_time = dt.datetime.today()
datelist = [cur_time - dt.timedelta(days=x) for x in range(0, 100)]

import pandas as pd
datelist = pd.date_range(pd.datetime.today(), periods=100).tolist()

date_formate = "%Y-%m-%d" # year-month-day
time = dt.strptime('2016-06-22', date_format)

time_str = dt.strftime("%Y-%m-%d", dt.now()) # return like "2016-06-22"