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Level 1 / Project 07 - Date Difference Helper

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Concept This project — Quiz Flashcards — Browser

Estimated time: 30 minutes

Focus

  • basic datetime parsing and deltas

Why this project exists

Compute differences between dates, add days to a date, and find the day of the week. You will learn the datetime module, strptime/strftime for parsing and formatting, and timedelta for date arithmetic.

Run (copy/paste)

Use <repo-root> as the folder containing this repository's README.md.

cd <repo-root>/projects/level-1/07-date-difference-helper
python project.py --input data/sample_input.txt
pytest -q

Expected terminal output

=== Date Helper ===

  diff 2024-01-01 2024-12-31 => 365 days
  diff 2024-06-15 2024-07-04 => 19 days
  add  2024-01-01 90         => 2024-03-31

Output written to data/output.json
6 passed

Expected artifacts

  • data/output.json
  • Passing tests
  • Updated notes.md

Checkpoint: Baseline code runs and all tests pass. Commit your work before continuing.

Alter it (required) — Extension

  1. Add a weeks command that returns the number of weeks and remaining days between two dates.
  2. Add support for alternate date formats like DD/MM/YYYY using a --format flag.
  3. Re-run script and tests.

Break it (required) — Core

  1. Add a line with an invalid date like diff 2024-02-30 2024-03-01 (Feb 30 does not exist) -- does parse_date() crash?
  2. Add a diff command with dates in reversed order -- does days_between() return a negative number?
  3. Add a command with a misspelled action like dif 2024-01-01 2024-01-10 -- what happens?

Fix it (required) — Core

  1. Wrap parse_date() in a try/except to handle invalid dates with a clear error message.
  2. Ensure days_between() always returns a non-negative value using abs().
  3. Add a test for the reversed-dates case.

Checkpoint: All modifications done, tests still pass. Good time to review your changes.

Explain it (teach-back)

  1. What does datetime.strptime("2024-01-15", "%Y-%m-%d") do and what does each % code mean?
  2. Why does timedelta(days=N) exist and how is it different from just adding an integer to a date?
  3. What does .strftime("%A") return and why is day-of-week useful?
  4. Where would date calculations appear in real software (billing cycles, SLA tracking, scheduling)?

Mastery check

You can move on when you can: - run baseline without docs, - explain one core function line-by-line, - break and recover in one session, - keep tests passing after your change.

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