The relativedelta type can be applied to an existing datetime and can replace secific components of that datetime. There are two ways to build a relativedelta instance: 

first, passing it two datetime classes
```
relativedelta(datetime1, datetime2)
```
second, passing it any number of the following keyword arguments
``` 
relativedelta(arg1=x, arg2=y, ...)
```
The order of attributtes considered when this relative data is added to a datetime is: 
Year
Month
Day
Hours
Minutes
Seconds
Microseconds

Using relativedelta

The rrule module offers a small, complete, and very fast, implementation of the recurrence rules documented in the iCalendar RFC.
The default constructor is:
```
rrule(freq)
```
where freq is one of YEARLY, MONTHLY, WEEKLY, DAILY, HOURLY, MINUTELY, or SECONDLY.

```dateutil.rrule.rrulestr(string)``` is a function that parses a string representation of a recurrence rule. 

rrule module

The tz module offers timezone implementations with subclasses to handle timezones such as local machine timezone, UTC timezone, given ranges, and TZ environment string. 

To create an instance using the UTC timezone, use: 
```
dateutil.tz.UTC
```

Functions

```datetutil.tz.gettz(name)``` retrieves a time zone object from a string representation. 
```dateutil.tz.datetime_ambiguous(dt, tz)``` determines whether or not a given datetime is ambiguous.
```dateutil.tz.datetime_exists(dt, tz-None)``` determines whether or not a give ndatetime wold fall in a gap.

tz module

https://dateutil.readthedocs.io/en/stable/

Dateutil documentation

The dateutil module provides powerful extensions to the standard datetime module, available in Python. Using the datetime module under the hood, this package allows: 
- Computing of relative deltas (next month, year, day of week etc.)
- Computing of dates based on recurrence rules
- Generic parsing of dates in string format
- Timezone implementations

dateutil can be installed from PyPI using pip
```
pip install python-dateutil
```

University of Michigan - Ann Arbor

- dateutil
- python imaging library (PIL / Pillow)
- sh

Useful modules/packages

https://wiki.python.org/moin/UsefulModules

List of useful modules and links to documentation

dateutil

The Python Imaging Library adds image processing capabilities to your Python interpreter. This library provides extensive file support, an efficient internal representation, and fairly powerful image processing capabilities. 

A few possible uses of this library includes
- Image Archives
- Image Display
- Image Processing 

pip can be installed with pip
```
pip install --upgrade pip
pip install -- upgrade Pillow
```

python imaging library (PIL / Pillow)

This package allows you to call any program as if it were a function. It works by running the binary commands on your system by dynamically resolving your $PATH, and then wrapping the binary in a function. 

To install sh, use the command:
```
pip install sh
```

Learn Before

Related

Learn After