Difference between dates (Not using Built-In Functions and Lists)

DiffBtwDates.py

#!/usr/bin/python
# This program is written without using any built-in functions
# Not using Lists, as a practice for students learning Python
def isLeapYear(year):
	if ((year%4 == 0 and year%100 != 0)) or year%400 == 0:
		return True
	return False

def daysInMonth(year, month):
	if month in {1,3,5,7,8,10,12}:
		days = 31
	if month in {4,6,9,11}:
		days = 30
	if month == 2:
		if isLeapYear(year):
			days = 29
		else:
			days = 28
	return days

def nextDay(year, month, day):
    if day < daysInMonth(year, month):
        return year, month, day + 1
    else:
        if month == 12:
            return year + 1, 1, 1
        else:
            return year, month + 1, 1
        
def dateIsBefore(year1, month1, day1, year2, month2, day2):
    """Returns True if year1-month1-day1 is before
       year2-month2-day2. Otherwise, returns False."""
    if year1 < year2:
        return True
    if year1 == year2:
        if month1 < month2:
            return True
        if month1 == month2:
            return day1 < day2
    return False        

def daysBetweenDates(year1, month1, day1, year2, month2, day2):
    """Returns the number of days between year1/month1/day1
       and year2/month2/day2. Assumes inputs are valid dates
       in Gregorian calendar."""
    # program defensively! Add an assertion if the input is not valid!
    assert not dateIsBefore(year2, month2, day2, year1, month1, day1)
    diff = 0
    while dateIsBefore(year1, month1, day1, year2, month2, day2):
        year1, month1, day1 = nextDay(year1, month1, day1)
        diff += 1
    return diff
print daysBetweenDates(2011,6,30,2012,6,30)