Python Datetime Summary

Datetime format is very frequently used in real world. This post summarized basic usage of datetime in python and pandas and introduced the dateutil library with examples.

Basic Python Datetime

The main module/objects of Datetime
are:

• datetime.datetime
  • datetime.date
  • datetime.time
  • datetime.datetime
• datetime.timedelta

datetime.datetime can be regarded as a combination of datetime.date and datetime.time. And datetime.timedelta is a standard format of time difference.

datetime.datetime

datetime.datetime contains the following attributes:
| Attribute | Directive | Class |
|———–|———–|——-|
| *Year | %Y | datetime.date |
| Month | %m | datetime.date |
| Day | %d | datetime.date |
| Hour | %H | datetime.time |
| Minute | %M | datetime.time |
| Second | %S | datetime.time |
| Microsecond | %f | datetime.time |

*: %Y is the year with the century (e.g: 1998, 1999), and %y is the year withour century (e.g: 98,99)

Datetime in Pandas

parse_time

parse_time is an option in pandas.read_csv(). If we set pd.read_csv(parse_time=[columns_name]), pandas will parse the chosen columns to datetime.datetime

Parse the time when read csv is quicker than pd.to_datetime. And you can a list of columns, which is very easy to use.

pd.to_datetime

It’s the usual way to convert a series of string to datetime.datetime. User can set the time format by a string of datetime attribute.
pd.to_datetime is slow. Be careful when you have a huge file.

pd.Grouper

Grouper is a very powerful tool to group the dataframe by any period of time, like 15 minutes, 1 hour or so on. It’s very useful when you need to analyse the data in small time scale.

Dateutil - Better Way to Calculate Timedelta

Dateutil is a useful extension library of python datetime. Sometimes, we need to calculate the date 3 month from the current date, or we need to calculate how many months between 2 dates. These problem is annoying because the days in each month are different. The dateutil is perfect for these applications.

Dateutil.relativedelta

When we need to calculate the relative date from one day (e.g. 3 day after today), relativedelta is the best choice.

Dateutil.rrule