# Count the Days Between Two Dates You are given two calendar dates as strings in `YYYY-MM-DD` format. Return the number of days between them as a non-negative integer. You must compute the result **without using any built-in date/time or calendar library** (e.g., no `datetime`, `java.time`, `Date`, or similar). The only calendar knowledge you may rely on is the standard Gregorian calendar rules given below. Assume a helper that returns the number of days in a given month is something you write yourself. ## Calendar rules - Months have the usual lengths: January 31, February 28 (29 in a leap year), March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, December 31. - A year is a **leap year** if it is divisible by 4, **except** years divisible by 100, **unless** they are also divisible by 400. So 2000 and 2024 are leap years; 1900 and 2100 are not. ## Input - `date1`: a string in `YYYY-MM-DD` format. - `date2`: a string in `YYYY-MM-DD` format. Both dates are guaranteed to be valid calendar dates between `1900-01-01` and `2100-12-31` inclusive. The dates may be given in either order (`date1` is not necessarily earlier than `date2`). ## Output Return a single integer: the absolute difference in days between the two dates. If the dates are equal, return `0`. ## Examples | date1 | date2 | Output | Explanation | |-------|-------|--------|-------------| | `2019-06-29` | `2019-06-30` | `1` | Consecutive days. | | `2019-12-31` | `2020-01-15` | `15` | Crosses a year boundary. | | `2020-02-28` | `2020-03-01` | `2` | 2020 is a leap year, so February 29 lies between them. | | `2019-02-28` | `2019-03-01` | `1` | 2019 is not a leap year. | | `2021-07-04` | `2021-07-04` | `0` | Same date. | ## Constraints - `1900-01-01 <= date1, date2 <= 2100-12-31` - Both inputs are valid dates in `YYYY-MM-DD` format. - No date/time library calls are allowed; leap-year handling must be implemented from the rules above.