Change time format in python dataframe

There is a difference between

• the content of a dataframe cell [a binary value] and
• its presentation [displaying it] for us, humans.

So the question is: How to reach the appropriate presentation of my datas without changing the data / data types themselves?

Here is the answer:

• If you use the Jupyter notebook for displaying your dataframe, or
• if you want to reach a presentation in the form of an HTML file [even with many prepared superfluous id and class attributes for further CSS styling — you may or you may not use them],

use styling. Styling don't change data / data types of columns of your dataframe.

Now I show you how to reach it in the Jupyter notebook — for a presentation in the form of HTML file see the note near the end of this answer.

I will suppose that your column DOB already has the datetime64 type [you have shown that you know how to reach it]. I prepared a simple dataframe [with only one column] to show you some basic styling:

• Not styled:

  df
  

          DOB
0  2019-07-03
1  2019-08-03
2  2019-09-03
3  2019-10-03

• Styling it as mm/dd/yyyy:

  df.style.format[{"DOB": lambda t: t.strftime["%m/%d/%Y"]}]
  

          DOB
0  07/03/2019
1  08/03/2019
2  09/03/2019
3  10/03/2019

• Styling it as dd-mm-yyyy:

  df.style.format[{"DOB": lambda t: t.strftime["%d-%m-%Y"]}] 
  

          DOB
0  03-07-2019
1  03-08-2019
2  03-09-2019
3  03-10-2019

Be careful!
The returning object is NOT a dataframe — it is an object of the class Styler, so don't assign it back to df:

Don't do this:

df = df.style.format[{"DOB": lambda t: t.strftime["%m/%d/%Y"]}]    # Don't do this!

[Every dataframe has its Styler object accessible by its .style property, and we changed this df.style object, not the dataframe itself.]

Questions and Answers:

• Q: Why your Styler object [or an expression returning it] used as the last command in a Jupyter notebook cell displays your [styled] table, and not the Styler object itself?

• A: Because every Styler object has a callback method ._repr_html_[] which returns an HTML code for rendering your dataframe [as a nice HTML table].

  Jupyter Notebook IDE calls this method automatically to render objects which have it.

Note:

You don't need the Jupyter notebook for styling [i.e., for nice outputting a dataframe without changing its data / data types].

A Styler object has a method render[], too, if you want to obtain a string with the HTML code [e.g., for publishing your formatted dataframe on the Web, or simply present your table in the HTML format]:

df_styler = df.style.format[{"DOB": lambda t: t.strftime["%m/%d/%Y"]}]
HTML_string = df_styler.render[]

strftime[format] 

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