convert strings of date to datetime exported from CSV

[buran]

you can use datetime module from standard library
or install and use some of the popular third-party packages that supposedly make it more convenient to work with dates. If it just for subtracting two dates built-in module should do

>>> from datetime import datetime
>>> s1='31/10/2019'
>>> s2='20/09/2019'
>>> d1 = datetime.strptime(s1, '%d/%m/%Y')
>>> d2 = datetime.strptime(s2, '%d/%m/%Y')
>>> date_diff = (d1 - d2).days
>>> date_diff
41