df[df.some_date.between(start_date, end_date)]
df[['A','B']] = df[['A','B']].apply(pd.to_datetime) #if conversion required
df['C'] = (df['B'] - df['A']).dt.days
# 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