pandas difference between dates

# credit to Stack Overflow user in the source link
import pandas as pd 
# df is your pandas dataframe
# if already datetime64 you don't need to use to_datetime

df['A'] = pd.to_datetime(df['A'])
df['B'] = pd.to_datetime(df['B']) 

df['diff'] =  df['A'] - df['B'] # difference in days

Here is what the above code is Doing:
1. Convert the columns to datetime
2. Subtract the columns
3. Convert the difference to days