Date Calculator App | Python Assignment

You will build a small but properly engineered Python CLI that computes date differences and future dates with timezone awareness. By the end you will have a tested tool with clean structure and packaging.

Learning Objectives

Structure a CLI app with argparse subcommands.
Do correct timezone-aware date math.
Cover the logic with pytest.
Time: ~6 hours · Difficulty: Intermediate · Prereqs: Python fundamentals.

Architecture Overview

graph LR CLI[argparse CLI] --> Core[date logic module] Core --> TZ[zoneinfo aware math] Tests[pytest] --> Core

Environment Setup

Python 3.11 in a venv with pytest.
A project layout with src/ and tests/.

Step-by-Step Execution

Implement the core date logic

from datetime import date, timedelta
def days_between(a: date, b: date) -> int:
    return abs((b - a).days)

Wire up the CLI

python -m datecalc diff 2026-06-17 2026-12-31

Run the tests

$ pytest -q

.... 4 passed in 0.05s

Progress So Far

graph LR A[01 Core logic] -->|done| B[02 CLI] B -->|done| C[03 Tests] style A fill:#1a4a1a,stroke:#00ff00,color:#fff style B fill:#1a4a1a,stroke:#00ff00,color:#fff style C fill:#1a4a1a,stroke:#00ff00,color:#fff

Testing & Validation

python -m datecalc diff 2026-06-17 2026-12-31 && pytest -q

The CLI should print 197 days and the test suite should pass. If both hold, the app is correct and shippable.

Troubleshooting

Off-by-one across DST: use timezone-aware datetimes for time-of-day math.
Import errors: run as a module (python -m) so package imports resolve.
Bad input crashes: validate date strings and exit non-zero with a message.

Extension Ideas

Package and publish to a private index.
Add the CLI conventions from CLI Tools with argparse.
Add CI with GitHub Actions.

Key Results

Shipped a working timezone-aware date CLI.
Covered the core logic with a passing pytest suite.
Separated logic, CLI, and tests cleanly.
Handled invalid input without crashing.

Date Calculator CLI Application in Python