Calculate duration between dates - OO

days_each_month = {1: 31, 2:28, 3:31, 4:30, 5:31, 6:30, 7:31, 8:31, 9:30, 10:31, 11:30, 12:31}

class Datetime(object): 
	def __init__(self, year, month, day): 
	    self.day = day
	    self.month = month
	    self.year = year 

class Package(object): 

	def __init__(self,date_shipped, date_delivered, include_startend_dates): 
	    self.date_shipped = date_shipped
	    self.date_delivered = date_delivered 
	    self.include_startend_dates = include_startend_dates
	
	@property 
	# adding 2 days includes the ship date and delivery date 
	def origin_dates(self):
		if self.include_startend_dates:
			return 2 
		else:
			return 0 
	
	@property 
	def number_of_years(self):
		return self.date_delivered.year - self.date_shipped.year
		
	def calculate_years(self):
		if self.number_of_years == 1 and self.date_shipped.month > self.date_delivered.month: 
			return 0
		else:
	  		return self.number_of_years * 365 
	
	def calculate_months(self):
		months = range(self.date_shipped.month, self.date_delivered.month +1)
		if self.date_shipped.month > self.date_delivered.month: 
			months = [k for k in days_each_month.keys() if k >= self.date_shipped.month or k <= self.date_delivered.month]
		days = 0 
		for month in months:
		  	days_in_month = days_each_month[month]		
		  	days += days_in_month 
		return days 
	
	def number_of_days_to_deliver(self):
		days_in_years = self.calculate_years() 
		
		origin_dates = self.origin_dates
			
		if self.number_of_years == 0 and self.date_shipped.month > self.date_delivered.month: 
			return 'Your end date cannot occur before your start date.' 
		else: 
			total_days_in_months_shipped = self.calculate_months()
			days_in_end_month = days_each_month[self.date_shipped.month] - self.date_delivered.day
			days_in_less_than_calendar_year = total_days_in_months_shipped - (self.date_shipped.day + days_in_end_month) 
		
		return days_in_years + days_in_less_than_calendar_year + origin_dates 
	
##############################################################
# TESTS
##############################################################

date_shipped_one = Datetime(year=2013, month=5, day=10) 
date_delivered_one = Datetime(year=2014, month=5, day=10)

date_shipped_two = Datetime(year=2013, month=5, day=10) 
date_delivered_two = Datetime(year=2016, month=5, day=10)

date_shipped_three = Datetime(year=2013, month=12, day=10) 
date_delivered_three = Datetime(year=2014, month=5, day=10)

p_one = Package(date_shipped=date_shipped_one, date_delivered=date_delivered_one, include_startend_dates=True)
one_year = p_one.number_of_days_to_deliver()

p_two = Package(date_shipped=date_shipped_two, date_delivered=date_delivered_two, include_startend_dates=True)
multiple_years = p_two.number_of_days_to_deliver()

p_three = Package(date_shipped=date_shipped_three, date_delivered=date_delivered_three, include_startend_dates=True)
off_calendar_half_year = p_three.number_of_days_to_deliver()

assert(one_year == 367)
assert(multiple_years == 1097)
assert(off_calendar_half_year == 153)