#returns the next day when given a date
def next_day(year,month,day):
    day = day + 1
    if day > in_month(year,month):
        day = 1
        month = month + 1
    if month > 12:
        month = 1
        year = year + 1
    return (year,month,day)

#returns the maximum day of a month
def in_month(year,month):
    if month in (4,6,9,11):
        month = 30
    elif month == 2 and is_leap_year(year) == True:
        month = 29
    elif month == 2:
        month = 28
    else:
        month = 31
    return month

#checks if a year is a leap year
def is_leap_year(year):
    if year % 4 == 0:
        if year % 100 == 0 and not year % 400 == 0:
            return False
        return True
    return False

#checks whether the first date occurs before the second
def date_before(year1,month1,day1,year2,month2,day2):
    if year1 < year2:
        return True
    if year1 == year2:
        if month1 < month2:
            return True
        if month1 == month2:
            if day1 < day2:
                return True
    return False

#calculates the number of days between two dates
def day_calculator(year1,month1,day1,year2,month2,day2):
    assert date_before(year1,month1,day1,year2,month2,day2) == True

    days = 0

    while date_before(year1,month1,day1,year2,month2,day2) == True:
        year1,month1,day1 = next_day(year1,month1,day1)
        days += 1
    return days