Days in month

Find how many days a month has, handle leap years like a pro.

datetime calendar leap-year date-math

Write a function `days_in_month(year: int, month: int) -> int` that returns the number of days in the given month (1–12) of the given year. Use the Gregorian calendar rules.

- The function must accept an integer `year` (>= 1) and an integer `month` from 1 (January) to 12 (December).
- It must return an integer representing the number of days in that month.
- The Gregorian leap year rule: a year is a leap year if it is divisible by 4, except if it is divisible by 100 but not by 400.

Do NOT use `calendar.monthrange`, `datetime`, or any external libraries.

Constraints

- `1 <= year <= 3000`
- `1 <= month <= 12`
- You may assume the input is always an integer within the given range.
- The function should return an integer.

Example

>>> days_in_month(2023, 2)
28
>>> days_in_month(2024, 2)
29
>>> days_in_month(2000, 2)
29
>>> days_in_month(1900, 2)
28

8 points ~10 min

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Hints

Create a list of days for each month, where February is 28 by default.

Handle the leap year for February: year % 4 == 0 and (year % 100 != 0 or year % 400 == 0).

Check the month boundary: if month is out of 1–12, what should happen? (The problem guarantees valid input, but you can still guard for safety.)

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