Date arithmetic (Timedelta) in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Timedelta in data analysis?

A Timedelta represents a duration or difference between two dates or times. It helps us calculate how much time has passed or will pass.

beginner

How do you add 5 days to a date using Timedelta in Python?

You create a Timedelta of 5 days and add it to the date. For example:
new_date = old_date + pd.Timedelta(days=5)

beginner

What happens when you subtract one date from another in pandas?

You get a Timedelta object that shows the difference in time between the two dates, like how many days or seconds apart they are.

beginner

How can Timedelta help in real life? Give an example.

Timedelta can help calculate delivery times, like how many days until a package arrives, or how long a project took by subtracting start and end dates.

intermediate

What units can Timedelta represent?

Timedelta can represent days, hours, minutes, seconds, milliseconds, microseconds, and nanoseconds.

What does pd.Timedelta(days=3) represent?

A A duration of 3 days

B A date 3 days ago

C The current date plus 3 days

D A time of 3 hours

pd.Timedelta(days=3) creates a duration of exactly 3 days, not a specific date or time.

If you subtract two dates in pandas, what type of object do you get?

A Datetime object

B Integer

C Timedelta object

D String

Subtracting two dates returns a Timedelta object representing the time difference.

How would you add 2 hours to a pandas Timestamp?

A timestamp + pd.Timedelta(hours=2)

B timestamp + 2

C timestamp + pd.Timedelta(days=2)

D timestamp + '2 hours'

You add a Timedelta with hours=2 to the timestamp to increase it by 2 hours.

Which of these is NOT a valid Timedelta unit?

A weeks

B months

C days

D seconds

Timedelta does not support months because months vary in length.

What will be the result of pd.Timestamp('2024-01-10') - pd.Timestamp('2024-01-01')?

A 1 day

B 10 days

C 0 days

D 9 days

The difference between Jan 10 and Jan 1 is 9 days.

Explain how you can use Timedelta to find the number of days between two dates.

Think about subtracting dates and checking the days in the result.

Describe how to add 3 hours and 30 minutes to a pandas Timestamp using Timedelta.

Use pd.Timedelta with hours and minutes parameters.

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