Pandas Time Series

Date and Time Data

Date and time data comes in several varieties
- Time stamps: reference particular moments in time
- Time intervals and periods: reference a length of time between a particular beginning and end point
- Time deltas or durations: reference an exact length of time

Dates and Times in Python

Python has a built-in datetime module for working with date and time data

>>> from datetime import datetime
>>> date = datetime(year=2020, month=11, day=13)
>>> date
datetime.datetime(2020, 11, 13, 0, 0)

The datetime object has many useful methods
Example: print the day of the week:
```
>>> date.strftime('%A')
'Friday'
```

Dates and Times in NumPy

NumPy has a native time series data types: datetime64 and timedelta64

>>> import numpy as np
>>> date = np.array('2020-11-13', dtype=np.datetime64)
>>> date
array('2020-11-13', dtype='datetime64[D]')

Vectorized operations are available for arrays of type datetime64

>>> date + np.arange(12)
array(['2020-11-13', '2020-11-14', '2020-11-15', '2020-11-16',
       '2020-11-17', '2020-11-18', '2020-11-19', '2020-11-20',
       '2020-11-21', '2020-11-22', '2020-11-23', '2020-11-24'],
      dtype='datetime64[D]')

Dates and Times in NumPy

NumPy datetime and timedelta64 data types are built on a fundamental time unit which imposes a trade-off between time resolution and maximum time span

>>> np.datetime64('2020-11-13')
numpy.datetime64('2020-11-13')
>>> np.datetime64('2020-11-13 12:00')
numpy.datetime64('2020-11-13T12:00')
>>> np.datetime64('2020-11-13 12:00:30.50', 'ns')
numpy.datetime64('2020-11-13T12:00:30.500000000')

NumPy Time Resolution Codes

Code	Meaning	Relative Time Span	Absolute Time Span
Y	Year	± 9.2e18 years	[9.2e18 BC, 9.2e18 AD]
M	Month	± 7.6e17 years	[7.6e17 BC, 7.6e17 AD]
W	Week	± 1.7e17 years	[1.7e17 BC, 1.7e17 AD]
D	Day	± 2.5e16 years	[2.5e16 BC, 2.5e16 AD]
h	Hour	± 1.0e15 years	[1.0e15 BC, 1.0e15 AD]
m	Minute	± 1.7e13 years	[1.7e13 BC, 1.7e13 AD]
s	Second	± 2.9e12 years	[ 2.9e9 BC, 2.9e9 AD]
ms	Millisecond	± 2.9e9 years	[ 2.9e6 BC, 2.9e6 AD]
us	Microsecond	± 2.9e6 years	[290301 BC, 294241 AD]
ns	Nanosecond	± 292 years	[ 1678 AD, 2262 AD]
ps	Picosecond	± 106 days	[ 1969 AD, 1970 AD]
fs	Femtosecond	± 2.6 hours	[ 1969 AD, 1970 AD]
as	Attosecond	± 9.2 seconds	[ 1969 AD, 1970 AD]

Dates and Times in Pandas

Pandas provides a Timestamp object which combines the ease-of-use of datetime with the efficient storage and vectorized interface of numpy.datetime64

Ease of parsing example

>>> import pandas as pd
>>> date = pd.to_datetime('13th of November, 2020')
>>> date
Timestamp('2020-11-13 00:00:00')
>>> date.strftime('%A')
'Friday'

Dates and Times in Pandas

Vectorization example

>>> date + pd.to_timedelta(np.arange(12), 'D')
DatetimeIndex(['2020-11-13', '2020-11-14', '2020-11-15', '2020-11-16',
               '2020-11-17', '2020-11-18', '2020-11-19', '2020-11-20',
               '2020-11-21', '2020-11-22', '2020-11-23', '2020-11-24'],
              dtype='datetime64[ns]', freq=None)

Pandas Time Series: Indexing by Time

A Series object can be indexed by time

>>> index = pd.DatetimeIndex(['2020-11-01', '2020-12-01',
...                           '2021-01-01', '2021-02-01'])
>>> data = pd.Series([0, 1, 2, 3], index=index)
>>> data
2020-11-01    0
2020-12-01    1
2021-01-01    2
2021-02-01    3
dtype: int64

Pandas Time Series: Indexing by Time

Indexing a time index

>>> data['2020-11-01':'2021-01-01']
2020-11-01    0
2020-12-01    1
2021-01-01    2
dtype: int64
>>> data['2020']
2020-11-01    0
2020-12-01    1
dtype: int64

Pandas Time Series Data Structures

timestamps: the Timestamp and DatetimeIndex
time periods: the Period and PeriodIndex
time deltas or durations: the Timedelta and TimedeltaIndex

Pandas Time Series Data Structures

Timestamp objects are commonly constructed with the to_datetime function

>>> dates = pd.to_datetime([datetime(2020, 11, 10), '11th of November, 2020',
...                        '2020-Nov-12', '11-13-2020', '20201114'])
>>> dates
DatetimeIndex(['2020-11-10', '2020-11-11', '2020-11-12', '2020-11-13',
               '2020-11-14'],
              dtype='datetime64[ns]', freq=None)

Pandas Time Series Data Structures

A DatetimeIndex can be converted to a PeriodIndex with the to_period function

>>> dates.to_period('D')
PeriodIndex(['2020-11-10', '2020-11-11', '2020-11-12', '2020-11-13',
             '2020-11-14'],
            dtype='period[D]', freq='D')

Pandas Time Series Data Structures

A TimedeltaIndex can be created when subtracting one date from another

>>> dates - dates[0]
TimedeltaIndex(['0 days', '1 days', '2 days', '3 days', '4 days'], dtype='timedelta64[ns]', freq=None)

Regular Sequences

Pandas has functions for creating regular date sequences:
- date_range
- period_range
- timedelta_range

date_range examples

>>> pd.date_range('2020-11-10', '2020-11-14')
DatetimeIndex(['2020-11-10', '2020-11-11', '2020-11-12', '2020-11-13',
               '2020-11-14'],
              dtype='datetime64[ns]', freq='D')
>>> pd.date_range('2020-11-10', periods=7)
DatetimeIndex(['2020-11-10', '2020-11-11', '2020-11-12', '2020-11-13',
               '2020-11-14', '2020-11-15', '2020-11-16'],
              dtype='datetime64[ns]', freq='D')

Regular Sequences

period_range example

>>> pd.period_range('2020-11-10', periods=7, freq='M')
PeriodIndex(['2020-11', '2020-12', '2021-01', '2021-02', '2021-03', '2021-04',
             '2021-05'],
            dtype='period[M]', freq='M')

timedelta_range example

>>> pd.timedelta_range(0, periods=10, freq='H')
TimedeltaIndex(['00:00:00', '01:00:00', '02:00:00', '03:00:00', '04:00:00',
                '05:00:00', '06:00:00', '07:00:00', '08:00:00', '09:00:00'],
               dtype='timedelta64[ns]', freq='H')

Frequencies and Offsets

Code	Description	Code	Description
`D`	Calendar day	`BD`	Business day
`W`	Weekly
`M`	Month end	`BM`	Business month end
`Q`	Quarter end	`BQ`	Business quarter end
`A`	Year end	`BA`	Business year end
`H`	Hours	`BH`	Business hours
`T`	Minutes
`S`	Seconds
`L`	Milliseconds
`U`	Microseconds
`N`	nanoseconds

Frequencies and Offsets (Continued)

The S suffix will change and end to a start

Code Description Code Description

MS Month start BMS Business month start

QS Quarter start BQS Business quarter start

AS Year start BAS Business year start
The month used to mark a quarterly or annual can be changed with a three-letter month sequence
- Q-JAN, BQ-FEB, QS-MAR, BQS-APR, etc.
- A-JAN, BA-FEB, QA-MAR, BQA-APR, etc.
The split-point of a weekly frequency can be modified with a three-letter weekday code
- W-SUN, W-MON, etc

Code	Description	Code	Description
`MS`	Month start	`BMS`	Business month start
`QS`	Quarter start	`BQS`	Business quarter start
`AS`	Year start	`BAS`	Business year start

Frequencies and Offsets Examples

Frequency of 2 hours and 30 minutes

>>> pd.timedelta_range(0, periods=9, freq='2H30T')
TimedeltaIndex(['00:00:00', '02:30:00', '05:00:00', '07:30:00', '10:00:00',
                '12:30:00', '15:00:00', '17:30:00', '20:00:00'],
               dtype='timedelta64[ns]', freq='150T')

Business day offsets

>>> pd.date_range('2020-11-13', periods=5, freq=BDay())
DatetimeIndex(['2020-11-13', '2020-11-16', '2020-11-17', '2020-11-18',
               '2020-11-19'],
              dtype='datetime64[ns]', freq='B')

Resampling, Shifting, and Windowing

Time series data can be resampled at a higher or lower frequency
- resample: data aggregation
- asfreq: data selection
Shifting data in time
- shift: shifts the data
- tshift: shifts the index
Rolling statistics are similar to the groupby operation on windows of time series data
- rolling