title: Get the nth day of a year
body: <p>For a homework assignment, I want to calculate the nth number of the day a date is in a year in a Jave program. </p>
  
  <p>So, a user gives a date, and then it tells what number of day this is in a year. So 1st of January 2019, is day 1.
  I already have a function that gives the number of days in a month. This function also considers leap years. 
  So I only need a function that returns the number of the day in the year. 
  What I thought, I had to do, but it does not work because I can't decrement a month:  </p>
  
  <pre><code>    static int dayNumberInYear(int day, Month month, int year)
      {
        while(month!=Month.JANUARY)
        {
          int dayNumberInYear=dayNumberInYear+numberOfDaysInMonth(year,month);
          Month month = month(-1);
        }
      return dayNumberInYear(day,month,year)+day;
      }
  </code></pre>
  
  <p>I know this is not right, so I hope someone can help me. 
  I think a for-loop is better, but I do not know how. And the first line, <code>static int dayNumberInYear(int day, Month month, int year)</code>, I am not allowed to change that, so it needs to be this first line. 
  <strong>I am not allowed to use the java JRE date manipulating classes like Calendar,
  Date etc!</strong></p>
  
  <p>I am a beginner, so please I hope that someone can help me.
  Here is the code I have so far:</p>
  
  <pre><code>    package ;
  
      import java.util.Scanner;
  
      public class Easter
      {
        public static void main(String[] arguments)
        {
          Scanner scanner=new Scanner(System.in);
          System.out.println("Enter the day month and year with spaces in between:");
          int day=scanner.nextInt();
          int monthNumber=scanner.nextInt();
          Month month=Month.month(monthNumber);
          int year=scanner.nextInt();
          System.out.println("The month "+month+" has "+numberOfDaysInMonth(year,month)+" days in year "+year);
  
          System.out.println("The number of this date in a year:"+dayNumberInYear(day,month,year));
  
          scanner.close();
        }
        static boolean isLeapYear(int year)
        {
          if(year%100==0)
            year=year/100;
          if(year%4==0)
            return true;
          return false;
        }
  
        static int numberOfDaysInMonth(int year, Month month)
        {
          switch(month)
          {
            case APRIL:
            case JUNE:
            case SEPTEMBER:
            case NOVEMBER:
              return 30;
            case FEBRUARY:
              if(isLeapYear(year))
                return 29;
              return 28;
            default:
              return 31;
          }
        }
  
        static int dayNumberInYear(int day, Month month, int year)
        {
          while(month!=Month.JANUARY)
          {
            int dayNumberInYear=dayNumberInYear+numberOfDaysInMonth(year,month);
            Month month = month(-1);
          }
          return dayNumberInYear(day,month,year)+day;
        }
      }
  </code></pre>
  
  <p>There is already a premade class Month.java:
          package ;</p>
  
  <pre><code>    public enum Month {
        JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER;
  
        public int number()
        {
          return ordinal()+1;
        }
  
        public static Month month(int number)
        {
          return values()[number-1];
        }
      }
  </code></pre>