今日學習了python用來建立日期索引的知識，方便以後進行時間序列操作。

import datetime

today = datetime.date.today()

yestoday = today - datetime.timedelta(days = 1)

關鍵函式 timedelta() 此函式用來進行兩個時間的加減，可以進行日，周，小時，分鐘，秒等的加減操作。

此外結合pandas的資料框建立，就可以創建出來類似於excel的表格式

date = pd.date_range(start='2020-10-01',end = yestoday)

如：df = pd.DataFrame(columns=['c1','c2'],index=date)

再通過for 迴圈就可以寫入資料了

for d in date:

　　a =dfxjl[dfxjl.日期==d].收款退款金額.sum()/10000

　　b = 計算資料

　　df.loc[d,:] = [a,b]

第一個關鍵函式：

Docstring:     
Difference between two datetime values.

timedelta(days=0, seconds=0, microseconds=0, milliseconds=0, minutes=0, hours=0, weeks=0)

All arguments are optional and default to 0.
Arguments may be integers or floats, and may be positive or negative.
第二個關鍵函式：
 

Signature:
pd.date_range(
    start=None,
    end=None,
    periods=None,
    freq=None,
    tz=None,
    normalize=False,
    name=None,
    closed=None,
    **kwargs,
) -> pandas.core.indexes.datetimes.DatetimeIndex
Docstring:
Return a fixed frequency DatetimeIndex.

Parameters
----------
start : str or datetime-like, optional
    Left bound for generating dates.
end : str or datetime-like, optional
    Right bound for generating dates.
periods : int, optional
    Number of periods to generate.
freq : str or DateOffset, default 'D'
    Frequency strings can have multiples, e.g. '5H'. See
    :ref:`here <timeseries.offset_aliases>` for a list of
    frequency aliases.
tz : str or tzinfo, optional
    Time zone name for returning localized DatetimeIndex, for example
    'Asia/Hong_Kong'. By default, the resulting DatetimeIndex is
    timezone-naive.
normalize : bool, default False
    Normalize start/end dates to midnight before generating date range.
name : str, default None
    Name of the resulting DatetimeIndex.
closed : {None, 'left', 'right'}, optional
    Make the interval closed with respect to the given frequency to
    the 'left', 'right', or both sides (None, the default).
**kwargs
    For compatibility. Has no effect on the result.

Returns
-------
rng : DatetimeIndex

See Also
--------
DatetimeIndex : An immutable container for datetimes.
timedelta_range : Return a fixed frequency TimedeltaIndex.
period_range : Return a fixed frequency PeriodIndex.
interval_range : Return a fixed frequency IntervalIndex.

Notes
-----
Of the four parameters ``start``, ``end``, ``periods``, and ``freq``,
exactly three must be specified. If ``freq`` is omitted, the resulting
``DatetimeIndex`` will have ``periods`` linearly spaced elements between
``start`` and ``end`` (closed on both sides).

To learn more about the frequency strings, please see `this link
<https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#offset-aliases>`__.

Examples
--------
**Specifying the values**

The next four examples generate the same `DatetimeIndex`, but vary
the combination of `start`, `end` and `periods`.

Specify `start` and `end`, with the default daily frequency.

>>> pd.date_range(start='1/1/2018', end='1/08/2018')
DatetimeIndex(['2018-01-01', '2018-01-02', '2018-01-03', '2018-01-04',
               '2018-01-05', '2018-01-06', '2018-01-07', '2018-01-08'],
              dtype='datetime64[ns]', freq='D')

Specify `start` and `periods`, the number of periods (days).

>>> pd.date_range(start='1/1/2018', periods=8)
DatetimeIndex(['2018-01-01', '2018-01-02', '2018-01-03', '2018-01-04',
               '2018-01-05', '2018-01-06', '2018-01-07', '2018-01-08'],
              dtype='datetime64[ns]', freq='D')

Specify `end` and `periods`, the number of periods (days).

>>> pd.date_range(end='1/1/2018', periods=8)
DatetimeIndex(['2017-12-25', '2017-12-26', '2017-12-27', '2017-12-28',
               '2017-12-29', '2017-12-30', '2017-12-31', '2018-01-01'],
              dtype='datetime64[ns]', freq='D')

Specify `start`, `end`, and `periods`; the frequency is generated
automatically (linearly spaced).

>>> pd.date_range(start='2018-04-24', end='2018-04-27', periods=3)
DatetimeIndex(['2018-04-24 00:00:00', '2018-04-25 12:00:00',
               '2018-04-27 00:00:00'],
              dtype='datetime64[ns]', freq=None)

**Other Parameters**

Changed the `freq` (frequency) to ``'M'`` (month end frequency).

>>> pd.date_range(start='1/1/2018', periods=5, freq='M')
DatetimeIndex(['2018-01-31', '2018-02-28', '2018-03-31', '2018-04-30',
               '2018-05-31'],
              dtype='datetime64[ns]', freq='M')

Multiples are allowed

>>> pd.date_range(start='1/1/2018', periods=5, freq='3M')
DatetimeIndex(['2018-01-31', '2018-04-30', '2018-07-31', '2018-10-31',
               '2019-01-31'],
              dtype='datetime64[ns]', freq='3M')

`freq` can also be specified as an Offset object.