timeDifference.py

# Take the difference of only the time portion of timestamps ignoring the date portion through dataframe columns.
import pandas as pd
import numpy as np
import datetime as dt
import datetime
from datetime import timedelta, datetime, date

df3 = pd.read_csv("combine.csv",  parse_dates=['bl.start','bl.end','ex.start','ex.end','ExamStart','ScrathcpadStart'])
# extracting time from timestamp

df3['bl.start1'] = df3['bl.start'].dt.time
df3['ExamStart2'] = df3['ExamStart'].dt.time

#It has looping to subtract two time portions.
#Take into account that looping through dataframes with pandas is NOT efficient way!
#Dont have too much time to dive into your problem but have a look at timestamp, timedelta, timedate.
#So you need to make it better!
df3['diff_seconds'] = [datetime.combine(date.min, df3['bl.start1'][ind]) - datetime.combine(date.min, df3['ExamStart2'][ind]) for ind, row in df3.iterrows()]
df3