Days Between Dates

Settlement systems live and die on date arithmetic: T+1 delivery, coupon accrual, day-count fractions. The library call is one line, so the interview version takes the library away: compute the exact number of days between two Gregorian calendar dates yourself.

Implement days_between(start, end) where both arguments are "YYYY-MM-DD" strings, returning the signed number of days from start to end (positive when end is later, negative when earlier, zero when equal). Do the calendar math yourself, without the datetime module or any other calendar library: the point of the exercise is the day-count algebra, datetime would trivialise it, and interviewers ask for exactly this without it. The editorial's follow-ups assume you did it by hand.

Gregorian leap rule: a year is a leap year if it is divisible by 4, except century years, which are leap only when divisible by 400. So 2000 and 2400 are leap years; 1900 is not.

Examples

days_between("2024-02-28", "2024-03-01")
# 2  (2024 is a leap year: Feb 29 sits in between)

days_between("2024-01-01", "2025-01-01")
# 366  (a leap year is 366 days long)

days_between("2026-03-01", "2026-02-01")
# -28  (signed: end before start goes negative)

Constraints

• Years range from 1583 to 9999 (safely inside the Gregorian calendar); inputs are always valid dates in YYYY-MM-DD form.
• The answer is exact integer arithmetic; there is nothing to round and no performance bar. The hidden tests are calendar edge cases: century non-leap years, the 400-year exceptions, and multi-millennium spans, so a hand-rolled month-by-month loop must get every rule right.