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

University of Michigan - Ann Arbor

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
```

dateutil

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.

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