Calculate Days Between Arbitrary Gregorian Dates

Problem Description

Write a function days_between(date1, date2) that returns the absolute number of days between two Gregorian dates. Each date is represented as a tuple or list: (year, month, day).

Rules

• A year is a leap year if it is divisible by 4, except for years divisible by 100 that are not divisible by 400.
• You must not iterate day-by-day. Your solution should ideally run in O(1) time complexity relative to the distance between dates.
• Years can be as large as 10,000,000.

Function Signature

def days_between(date1: tuple, date2: tuple) -> int:
    # date = (year, month, day)

Example

# Input: (2020, 1, 1), (2021, 1, 1)
# Output: 366
# Explanation: 2020 was a leap year.

# Input: (2000, 1, 1), (2001, 1, 1)
# Output: 366
# Explanation: 2000 is divisible by 400, thus a leap year.

Constraints

• 1 <= year <= 10^7
• 1 <= month <= 12
• 1 <= day <= days_in_month(month, year)

Edge Cases

• Leap years at the turn of the century (e.g., 1900 vs 2000).
• Same date inputs (should return 0).
• Dates in the same year/same month.