df = pd.DataFrame({'TimeWeek': ['6:40:00', '6:45:00', '6:50:00', '6:55:00',
    '7:40:00']})
df['TimeWeek_date'] = pd.to_datetime(df['TimeWeek'], format='%H:%M:%S')
print(df)
df['TimeWeekDiff'] = df['TimeWeek_date'] - df['TimeWeek_date'].shift().fillna(
    pd.to_datetime('00:00:00', format='%H:%M:%S'))
print(df)
Calculate difference between 'times' rows in DataFrame Pandas