Find remain day in a year with no import datatime in Python

I write a code which is to find the remaining day and passed days in a year. I have to use for loop for my homework. This code can be true sometimes but such as day:29 month:12 year:2020 dates
program find wrong left days and passed days. Where is the mistake on my code ı couldn’t understand.

 day = int(input("Enter a day: "))
month = int(input("Enter a month: "))
year = int(input("Enter a year: "))
month_day = None
days_passed = 0
days_left = None

if year % 4 == 0 and year > 0:
    if year % 100 != 0:
        year = "Leap year"

    elif year % 100 == 0:
        if year % 400 == 0:
            year = "Leap year"
        else:
            year = "Not leap year"

elif year <= 0:
    print("Please enter a valid number")

elif year % 4 != 0 and year > 0:
    year = "Not leap year"

elif month < 1 and month > 12:
    print("Please enter a valid number")

if year == "Leap year":
    if month == 2:
        month_day = 29
    elif month == 1 or month == 3 or month == 5 or month == 7 or month == 8 or month == 10 or month == 12:
        month_day = 31
    else:
        month_day = 30

elif year == "Not leap year":
    if month == 2:
        month_day = 28
    elif month == 1 or month == 3 or month == 5 or month == 7 or month == 8 or month == 10 or month == 12:
        month_day = 31
    else:
        month_day = 30

if year == "Leap year":
    for i in range(1,month):
        days_passed += month_day
        month -= 1
        if month == 0:
            break
    days_passed += day
    days_left = 366- days_passed

    print("Days passed: ", days_passed)
    print("Days left: ",days_left)




if year == "Not leap year":
    for i in range(1, month):
        days_passed += month_day
        month -= 1
        if month == 1:
            break
    days_passed += day
    days_left = 365 - days_passed

    print("Days passed: ", days_passed )
    print("Days left: ", days_left)

>Solution :

Without the aid of standard modules, for loops or dictionaries, you could do it like this:

def isleap(y):
    if y % 400 == 0:
        return True
    if y % 100 == 0:
        return False
    return y % 4 == 0


def elapsed(d, m, y):
    assert y > 0
    assert m > 0 and m < 13
    assert d > 0
    dim = [0, 31, 0, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    dim[2] = 29 if isleap(y) else 28
    assert d <= dim[m]
    return sum(dim[0:m]) + d


print(elapsed(1, 3, 1980))