The Calendar Sort

An easy Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain: Python
Difficulty: easy
Seniority: L3

Problem

Given a list of date strings 'YYYY-MM-DD', return them sorted chronologically ascending. Since ISO format sorts correctly as strings, a lexicographic sort works. Do not use date libraries.

Time has its own opinion about order.

Example

Input:

dates = ["2024-03-15","2023-01-01","2024-01-01"]

Output:

["2023-01-01","2024-01-01","2024-03-15"]

Worked solution and explanation

Why this problem exists in real interviews

Sorting date strings lexicographically tests the insight that YYYY-MM-DD format sorts correctly as strings. No date parsing is needed because the format is already year-major.

Break down the requirements

Step 1: Recognize that ISO date format sorts lexicographically

The YYYY-MM-DD format is designed so that string comparison matches chronological order.

Step 2: Sort the list of strings

A standard string sort produces the correct chronological order.

The solution

Lexicographic sort exploiting ISO date format

def sort_dates(dates):
    result = sorted(dates)
    return result

Common follow-up questions

What if the dates were in MM/DD/YYYY format? (Tests parsing to a comparable format before sorting, since month-first does not sort correctly as strings.)
What if some dates have single-digit months without zero-padding? (Tests that `'2024-1-15'` sorts incorrectly vs. `'2024-01-15'` and requires normalization.)
How would you sort dates with timestamps? (Tests that ISO 8601 with `T` separator also sorts correctly as strings.)