public class Date {

	private int day;
	private int month;
	private int year;

	public Date(int year, int month, int day) {
		if (!validDate(year, month, day)) {
			throw new IllegalArgumentException("Not a valid date");
		}
		this.day = day;
		this.month = month;
		this.year = year;
	}

	public void incrementOneDay() {
		day += 1;
		if (day == 32 && month == 1) {
			day = 1;
			month = 2;

		} else if (day == 29 && month == 2 && !isLeapyear(year)) {
			day = 1;
			month = 3;
		} else if (day == 30 && month == 2 && isLeapyear(year)) {
			day = 1;
			month = 3;
		} else if (day == 32 && month == 3) {
			day = 1;
			month = 4;
		} else if (day == 31 && month == 4) {
			day = 1;
			month = 5;
		} else if (day == 32 && month == 5) {
			day = 1;
			month = 6;
		} else if (day == 31 && month == 6) {
			day = 1;
			month = 7;
		} else if (day == 32 && month == 7) {
			day = 1;
			month = 8;
		} else if (day == 32 && month == 8) {
			day = 1;
			month = 9;
		} else if (day == 31 && month == 9) {
			day = 1;
			month = 10;
		} else if (day == 32 && month == 10) {
			day = 1;
			month = 11;
		} else if (day == 31 && month == 11) {
			day = 1;
			month = 12;
		} else if (day == 32 && month == 12) {
			day = 1;
			month = 1;
			year += 1;
		}

	}

	public void incrementNDays(int n) {
		for (int i = 1; i <= n; i++) {
			incrementOneDay();
		}
	}

	public void decrementOneDay() {
		day -= 1;
		if (day == 0) {
			if (month == 1) {
				day = 31;
				month = 12;
				year += -1;
			} else if (month == 2) {
				day = 31;
				month = 1;
			} else if (month == 3) {
				if (isLeapyear(year)) {
					day = 29;
					month = 2;
				} else {
					day = 28;
					month = 2;
				}
			} else if (month == 4) {
				day = 31;
				month = 3;
			} else if (month == 5) {
				day = 30;
				month = 4;
			} else if (month == 6) {
				day = 31;
				month = 5;
			} else if (month == 7) {
				day = 30;
				month = 6;
			} else if (month == 8) {
				day = 31;
				month = 7;
			} else if (month == 9) {
				day = 31;
				month = 8;
			} else if (month == 10) {
				day = 30;
				month = 9;
			} else if (month == 11) {
				day = 31;
				month = 10;
			} else if (month == 12) {
				day = 30;
				month = 11;
			}
		}
	}

	public void decrementNDays(int n) {
		for (int i = 1; i <= n; i++) {
			decrementOneDay();
		}
	}

	public static boolean isLeapyear(int year) {
		return ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0);
	}

	public static int daysBetweenDates(Date dateOne, Date dateTwo) {
		int numDays = 0;
		boolean date1Smaller = true;
		Date date1 = new Date(dateOne.getYear(), dateOne.getMonth(),
				dateOne.getDay());
		Date date2 = new Date(dateTwo.getYear(), dateTwo.getMonth(),
				dateTwo.getDay());

		if (date1.getYear() > date2.getYear()) {
			date1Smaller = false;
		} else if (date1.getYear() < date2.getYear()) {
			date1Smaller = true;
		} else if (date1.getMonth() < date2.getMonth()) {
			date1Smaller = true;
		} else if (date1.getMonth() > date2.getMonth()) {
			date1Smaller = false;
		} else if (date1.getDay() < date2.getDay()) {
			date1Smaller = true;
		} else if (date1.getDay() > date2.getDay()) {
			date1Smaller = false;
		} else {
			return 0;
		}

		while (!date1.toString().equals(date2.toString())) {
			numDays++;
			if (date1Smaller) {
				date1.incrementOneDay();
			} else {
				date1.decrementOneDay();
			}
		}

		return numDays;
	}

	public static boolean validDate(int year, int month, int day) {
		if (day < 1 || day > 31) {
			return false;
		}
		if (month < 1 || month > 12) {
			return false;
		}
		if (year < 0) {
			return false;
		}
		if (day == 31
				&& (month == 2 || month == 4 || month == 6 || month == 9 || month == 11)) {
			return false;
		}
		if (day > 28 && month == 2 && !isLeapyear(year)) {
			return false;
		}
		if (day > 29 && month == 2 && isLeapyear(year)) {
			return false;
		}

		return true;
	}

	public int getMonth() {
		return month;
	}

	public int getYear() {
		return year;
	}

	public int getDay() {
		return day;
	}

	public String toString() {
		String dateString = year + "/" + month + "/" + day;
		return dateString;
	}
}