date1.py

def nextDay(year, month, day):
    """Simple version: assume every month has 30 days"""
    if day < 30:
        return year, month, day + 1
    else:
        if month == 12:
            return year + 1, 1, 1
        else:
            return year, month + 1, 1

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, and the first date is not after
       the second."""

    """To do this we need to count up from year1,month1,day1 until
       we get to year2,month2,day2 and track how far we counted.
       """

    """To make things clearer, lets make a copy of year1, month1,
       and day1 so that we aren't re-using variables, this is just
       a trick to make the code easier to read and maintain."""

    year_counter = year1
    month_counter = month1
    day_counter= day1
    # ok, now year_counter, month_counter and day_counter are the same
    # as year1, month1, and day1.

    """Ok, now what we are doing to do is loop over and over using
       nextDay() to add to year_counter, month_counter and day_counter,
       until year_counter, month_counter and day_counter equal
       year2,month2,day2, while doing that we will use the variable
       'count' to count how many times we loop because that will
       indicate how many days."""

    count = 0 # count needs to start at zero
    while [year_counter, month_counter, day_counter] != [year2, month2, day2]:
        """year_counter, month_counter, day_counter is not equal to year2, month2, day2
           so we..."""

        # add one to 'count'
        count = count + 1

        # update year_counter, month_counter, day_counter to be the next day
        year_counter, month_counter, day_counter = nextDay(year_counter, month_counter, day_counter)

        """now we've reached the end of our loop, so we will jump back to the "while" above
           and try again until the comparison is false."""

    """give back how many times we looped, which is how many days there are different between the
    two."""

    return count

def daysBetweenDates2(year1, month1, day1, year2, month2, day2):
    """Same code as before, just the while loop is written so maybe it is
       easier to read."""

    year_counter = year1
    month_counter = month1
    day_counter= day1

    count = 0 # count needs to start at zero

    """ What we will do is loop forever until told to stop, that's what "while True:" does.
        a call to "break" will exit the loop."""
    while True: # loop until we say stop

        # if they are the same, then escape from this loop
        if year_counter == year2 and month_counter == month2 and day_counter == day2:
               break # get me out of the while loop!

        # otherwise add one to 'count'
        count = count + 1

        # update year_counter, month_counter, day_counter to be the next day
        year_counter, month_counter, day_counter = nextDay(year_counter, month_counter, day_counter)

    return count

def daysBetweenDates3(year1, month1, day1, year2, month2, day2):
    """Same code as before, just the while loop is written so maybe it is
       easier to read, also I changed the way we get our data back from
       nextDay() so that maybe that is also easier to read."""

    year_counter = year1
    month_counter = month1
    day_counter= day1

    count = 0 # count needs to start at zero

    """ What we will do is loop forever until told to stop, that's what "while True:" does.
        a call to "break" will exit the loop."""
    while True: # loop until we say stop

        # if they are the same, then escape from this loop
        if year_counter == year2 and month_counter == month2 and day_counter == day2:
               break # get me out of the while loop!

        # otherwise add one to 'count'
        count = count + 1

        """So maybe the way we assign multiple variables coming back from nextDay() looks
           weird, so instead let's just get back the "array" from nextDay and assign it
           in another step so it's more clear how the data is coming back."""
        # tomorrow is now an array like so [tomorrow_year, tomorrow_month, tomorrow_day]
        tomorrow_array = nextDay(year_counter, month_counter, day_counter)
        # now extract each element from the array into our variables
        year_counter = tomorrow_array[0] # first element is year
        month_counter = tomorrow_array[1] # next is month
        day_counter = tomorrow_array[2] # third is day

    return count


def daysBetweenDates_fast(year1, month1, day1, year2, month2, day2):
    days = day2 - day1
    # we subtracted and if days < 0 then we just have to "borrow" from the next
    # digit (the months) just like in normal grade school arithmatic
    if days < 0:
        """let's say we got -7, then we need to reset days to "23" and take away a month
           how do we do that?
           We add our negative days to 30 and we should get 23, right?
           Then we just have to borrow a 1 from the months!"""
        days = 30 + days
        month2 = month2 - 1

    """ok now let's do the month, pretty much the same as days, except the range is not 30, it's 12
       and we borrow from years if needed"""
    months = month2 - month1
    if months < 0:
        months = 12 + month
        year2 = year2 - 1

    # ok now just subtract the years
    years = year2 - year1

    """So now we have the different sorted out, but it's in years, months and days.
       We need to turn it into days so let's just multiply things out."""
    # first move all the years into the months:
    # now we have "months + days"
    months = months + (years * 12)
    # now let's convert those months into days:
    days = days + (months * 30)

    return days

def daysBetweenDates_fast_easy(year1, month1, day1, year2, month2, day2):
    """ Let's try one more super easy way...
        Let's just convert both dates into "days" and then subtract them, SO EASY!!!"""

    total_days1 = (year1 * 12 * 30) + (month1 * 30) + day1
    total_days2 = (year2 * 12 * 30) + (month2 * 30) + day2

    return total_days2 - total_days1



# Let's try a few times:


print "Number of days between 2015 and 2016 is: {}".format(daysBetweenDates(2015, 1, 1, 2016, 1, 1))
print "Number of days between Jan 1, 2015 and Jan 15, 2015 is: {}".format(
        daysBetweenDates(2015, 1, 1, 2015, 1, 15))
print "Number of days between Jan 1, 1980 and Jan 15, 2015 is: {}".format(
        daysBetweenDates(1980, 1, 1, 2015, 1, 15))


# using the second function
print "2nd function: Number of days between Jan 1, 1980 and Jan 15, 2015 is: {}".format(
        daysBetweenDates2(1980, 1, 1, 2015, 1, 15))

# using the third function
print "3rd function: Number of days between Jan 1, 1980 and Jan 15, 2015 is: {}".format(
        daysBetweenDates3(1980, 1, 1, 2015, 1, 15))

print "fast: Number of days between 2015 and 2016 is: {}".format(daysBetweenDates_fast(2015, 1, 1, 2016, 1, 1))
print "fast: Number of days between Jan 1, 2015 and Jan 15, 2015 is: {}".format(
        daysBetweenDates_fast(2015, 1, 1, 2015, 1, 15))
print "fast: Number of days between Jan 1, 1980 and Jan 15, 2015 is: {}".format(
        daysBetweenDates_fast(1980, 1, 1, 2015, 1, 15))

print "fast_easy: Number of days between 2015 and 2016 is: {}".format(daysBetweenDates_fast_easy(2015, 1, 1, 2016, 1, 1))
print "fast_easy: Number of days between Jan 1, 2015 and Jan 15, 2015 is: {}".format(
        daysBetweenDates_fast_easy(2015, 1, 1, 2015, 1, 15))
print "fast_easy: Number of days between Jan 1, 1980 and Jan 15, 2015 is: {}".format(
        daysBetweenDates_fast_easy(1980, 1, 1, 2015, 1, 15))