int fine=0;

            if(returned_y<due_y){
                fine = 0; 
            } else if(returned_y>due_y){
                fine = 10000;
            } else if(returned_m>due_m){
                    fine = (returned_m-due_m)*500;
            } else if(returned_d>due_d){
                    fine = (returned_d-due_d)*15;
            }
        System.out.println(fine);

Constant time procedure using Java:

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    Queue <Integer> myQueue = new LinkedList<Integer>();

    for (int i = 0; i < 3; i++) {
        myQueue.add(input.nextInt());
    }

    int dif1 = myQueue.remove() - input.nextInt();
    int dif2 = myQueue.remove() - input.nextInt();
    int dif3 = myQueue.remove() - input.nextInt();

    if ((dif1 <= 0 && dif2 <= 0 && dif3 <= 0) || (dif1 > 0 && (dif2 < 0 || dif3 < 0)))
        System.out.println(0);
    else if (dif1 > 0 && dif2 == 0 && dif3 == 0) 
        System.out.println(15*dif1);
    else if (dif2 > 0 && dif3 == 0) 
        System.out.println(500*dif2);
    else 
        System.out.println(10000);

}

C++ code

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    int D,M,Y,d,m,y,fine=0;
    cin>>D>>M>>Y;
    cin>>d>>m>>y;
    if(D>d&&M==m&&y==Y)
        fine=15*(D-d);
    else if(M>m&&y==Y)
        fine=500*(M-m);
    else if(Y>y)
        fine=10000;
    
    cout<<fine;
    
    
    return 0;
}

you dont need anything else than some nested if n else.

date,month,year = map(int,input().strip().split())
DueDate,DueMonth,DueYear = map(int,input().strip().split())

DayLate = date-DueDate
monthLate = month-DueMonth
yearLate = year-DueYear

if yearLate==0:
    if monthLate==0:
        if DayLate<=0:
            print(0)
        else:
            print(DayLate*15)
    elif monthLate < 0 :
        print(0)
    else:
        print(500*monthLate)
elif yearLate < 0:
    print(0)
else:
    print(10000)

The test case #6 has inputs: 8 4 12 1 1 1 and the expected output is: 10000

but the difference between the years is 12-1 = 11 and the fine will be: fine = 11 * 10000 = 110000

this test case itself is wrong