Date arithmetic (Timedelta) in Data Analysis Python - Mini Project: Build & Apply

Date Arithmetic with Timedelta in Python

📖 Scenario: Imagine you work in a library. You have a list of books with their borrow dates. You want to find out when each book should be returned by adding a fixed borrowing period.

🎯 Goal: You will create a dictionary of books with their borrow dates, set a borrowing period, calculate the return dates by adding the borrowing period to each borrow date, and finally display the return dates.

📋 What You'll Learn

Create a dictionary with book titles as keys and borrow dates as datetime objects as values

Create a variable for the borrowing period using timedelta

Calculate return dates by adding the borrowing period to each borrow date using a dictionary comprehension

Print the dictionary of return dates

💡 Why This Matters

🌍 Real World

Libraries, rental services, and any system that tracks borrowed items use date arithmetic to calculate due dates.

💼 Career

Understanding date arithmetic is important for data analysts and scientists working with time series data, scheduling, and event tracking.

1

Create the dictionary of borrow dates

Create a dictionary called borrow_dates with these exact entries: 'Book A': datetime(2024, 6, 1), 'Book B': datetime(2024, 6, 3), 'Book C': datetime(2024, 6, 5). Import datetime from the datetime module.

Data Analysis Python

from datetime import datetime
# Create the borrow_dates dictionary with exact entries
# Your code here

Hint

Use datetime(2024, 6, 1) to create the date for June 1, 2024.

2

Set the borrowing period

Import timedelta from the datetime module and create a variable called borrow_period that represents 14 days.

Data Analysis Python

from datetime import datetime, timedelta
borrow_dates = {
    'Book A': datetime(2024, 6, 1),
    'Book B': datetime(2024, 6, 3),
    'Book C': datetime(2024, 6, 5)
}
# Create borrow_period as timedelta of 14 days
# Your code here

Hint

Use timedelta(days=14) to represent 14 days.

3

Calculate return dates using dictionary comprehension

Create a new dictionary called return_dates using a dictionary comprehension. For each book and borrow_date in borrow_dates.items(), add borrow_period to borrow_date.

Data Analysis Python

from datetime import datetime, timedelta
borrow_dates = {
    'Book A': datetime(2024, 6, 1),
    'Book B': datetime(2024, 6, 3),
    'Book C': datetime(2024, 6, 5)
}
borrow_period = timedelta(days=14)
# Create return_dates dictionary by adding borrow_period to each borrow_date
# Your code here

Hint

Use {book: borrow_date + borrow_period for book, borrow_date in borrow_dates.items()} to create the new dictionary.

4

Print the return dates

Print the return_dates dictionary to see the calculated return dates.

Data Analysis Python

from datetime import datetime, timedelta
borrow_dates = {
    'Book A': datetime(2024, 6, 1),
    'Book B': datetime(2024, 6, 3),
    'Book C': datetime(2024, 6, 5)
}
borrow_period = timedelta(days=14)
return_dates = {book: borrow_date + borrow_period for book, borrow_date in borrow_dates.items()}
# Print the return_dates dictionary
# Your code here

Hint

Use print(return_dates) to display the dictionary.

Practice

1. What does the timedelta object represent in Python's date arithmetic?

easy

A. A method to convert strings to dates

B. A specific date and time value

C. A function to format dates

D. A duration or difference between two dates or times

Solution

1. Step 1: Understand the purpose of timedelta

   The timedelta object is used to represent a span of time, such as days, seconds, or microseconds.

2. Step 2: Differentiate timedelta from other date/time objects

   Unlike a specific date or time, timedelta measures the difference or duration between two dates or times.

3. Final Answer:

   A duration or difference between two dates or times -> Option D

4. Quick Check:

   timedelta = duration/difference [OK]

Hint: timedelta means time difference or duration [OK]

Common Mistakes:

• Confusing timedelta with a specific date
• Thinking timedelta formats dates
• Assuming timedelta converts strings

2. Which of the following is the correct way to create a timedelta of 5 days in Python?

easy

A. timedelta(5 days)

B. timedelta(seconds=5)

C. timedelta(days=5)

D. timedelta(day=5)

Solution

1. Step 1: Recall timedelta constructor syntax

   The timedelta constructor accepts keyword arguments like days, seconds, etc.

2. Step 2: Identify the correct keyword and format

   timedelta(5 days): syntax error ('days' undefined). timedelta(seconds=5): 5 seconds not days. timedelta(day=5): invalid keyword 'day'. timedelta(days=5): correct days=5.

3. Final Answer:

   timedelta(days=5) -> Option C

4. Quick Check:

   Use keyword days=5 for 5 days [OK]

Hint: Use keyword days=number to create timedelta days [OK]

Common Mistakes:

• Using 'days' without quotes causing NameError
• Using seconds=5 instead of days=5
• Misspelling 'days' as 'day'

3. What will be the output of this code?

from datetime import datetime, timedelta
start = datetime(2024, 6, 10)
new_date = start + timedelta(days=7)
print(new_date.strftime('%Y-%m-%d'))

medium

A. 2024-06-17

B. 2024-06-03

C. 2024-07-10

D. 2024-06-10

Solution

1. Step 1: Understand the initial date

   The variable start is June 10, 2024.

2. Step 2: Add 7 days using timedelta

   Adding 7 days to June 10 results in June 17, 2024.

3. Step 3: Format the date output

   The strftime formats the date as 'YYYY-MM-DD', so output is '2024-06-17'.

4. Final Answer:

   2024-06-17 -> Option A

5. Quick Check:

   June 10 + 7 days = June 17 [OK]

Hint: Add days to date, then format output string [OK]

Common Mistakes:

• Subtracting days instead of adding
• Misreading date format
• Confusing month and day positions

4. Identify the error in this code snippet:

from datetime import datetime, timedelta
start = datetime(2024, 6, 10)
new_date = start + timedelta(day=7)
print(new_date)

medium

A. timedelta uses invalid keyword 'day'

B. datetime object cannot be added to timedelta

C. print statement syntax is incorrect

D. datetime constructor missing time parameters

Solution

1. Step 1: Check timedelta constructor usage

   The timedelta constructor requires keyword arguments like days=7; day=7 is invalid keyword causing TypeError.

2. Step 2: Verify other parts of code

   Adding timedelta to datetime is valid, print syntax is correct, and time parameters are optional.

3. Final Answer:

   timedelta uses invalid keyword 'day' -> Option A

4. Quick Check:

   timedelta(days=7) needed, not day=7 [OK]

Hint: Always use keyword days= for timedelta days [OK]

Common Mistakes:

• Misspelling 'days' as 'day'
• Thinking datetime + timedelta is invalid
• Assuming print syntax error

5. You have a DataFrame with a column 'start_date' of type datetime. You want to create a new column 'deadline' that is 10 days after 'start_date'. Which code correctly does this using pandas and timedelta?

import pandas as pd
from datetime import timedelta

df = pd.DataFrame({'start_date': pd.to_datetime(['2024-06-01', '2024-06-05'])})
# Which line correctly adds 10 days?

hard

A. df['deadline'] = df['start_date'] + timedelta(seconds=10)

B. df['deadline'] = df['start_date'] + pd.Timedelta(days=10)

C. df['deadline'] = df['start_date'] + timedelta(day=10)

D. df['deadline'] = df['start_date'] + pd.timedelta(days=10)

Solution

1. Step 1: Understand pandas datetime addition

   When adding time to pandas datetime columns, use pd.Timedelta for vectorized operations.

2. Step 2: Check each option

   df['deadline'] = df['start_date'] + pd.Timedelta(days=10): correct pd.Timedelta(days=10). df['deadline'] = df['start_date'] + timedelta(seconds=10): timedelta(seconds=10) adds seconds not days. df['deadline'] = df['start_date'] + timedelta(day=10): invalid keyword day=10. df['deadline'] = df['start_date'] + pd.timedelta(days=10): pd.timedelta doesn't exist.

3. Final Answer:

   df['deadline'] = df['start_date'] + pd.Timedelta(days=10) -> Option B

4. Quick Check:

   Use pd.Timedelta for pandas datetime addition [OK]

Hint: Use pd.Timedelta(days=10) to add days to pandas datetime [OK]

Common Mistakes:

• Using seconds=10 instead of days=10
• Using invalid keyword 'day=10'
• Wrong capitalization pd.timedelta