1. 计算给出两个时间之间的时间差
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
2. 获取n天前的时间
cur_time = dt.now()
# previous n days
pre_time = dt.timedelta(days=n)
3. 将给定的时间精确到天或者其他单位
cur_time = dt.now()
# get day of current time
cur_day = cur_time.replace(hour=0, minute=0, second=0, mircrosecond=0)
4. 获取一连串的时间序列（返回list）
cur_time = dt.datetime.today()
datelist = [cur_time - dt.timedelta(days=x) for x in range(0, 100)]
或者
import pandas as pd
datelist = pd.date_range(pd.datetime.today(), periods=100).tolist()
5. 将时间字符串转化为datetime类型
date_formate = "%Y-%m-%d" # year-month-day
time = dt.strptime('2016-06-22', date_format)
6. 将时间类型转化为字符串类型
time_str = dt.strftime("%Y-%m-%d", dt.now()) # return like "2016-06-22"
参考：
https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior
http://stackoverflow.com/questions/993358/creating-a-range-of-dates-in-python
http://stackoverflow.com/questions/151199/how-do-i-calculate-number-of-days-betwen-two-dates-using-python
http://stackoverflow.com/questions/3183707/stripping-off-the-seconds-in-datetime-python