Get the nth day of a year

For a homework assignment, I want to calculate the nth number of the day a date is in a year in a Jave program.

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:

    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;
    }

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, static int dayNumberInYear(int day, Month month, int year), I am not allowed to change that, so it needs to be this first line. I am not allowed to use the java JRE date manipulating classes like Calendar, Date etc!

I am a beginner, so please I hope that someone can help me. Here is the code I have so far:

    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;
      }
    }

There is already a premade class Month.java: package ;

    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];
      }
    }

It looks like the idea you're going for is a recursive approach that decrements by month.

static int dayNumberInYear(int day, Month month, int year)
    {
      if(!month.equals(Month.JANUARY)) {
        return day + dayNumberInYear(numberOfDaysInMonth(month.minus(1), year), month.minus(1), year);
      } else {
        return day;
      }
}

Note that in this recursive approach, the base case is that it's January and we simply return the current day in January. Otherwise we add the day of the current month and then all the days in every prior month.

It could also be done as a for-loop.

static int dayNumberInYearByLoop(int day, Month month, int year) {
  int totalDays = day;
  for (int i = month.getValue();i>1;i--) {
    totalDays += numberOfDaysInMonth(Month.of(i), year);
  }
  return totalDays;
}

You can mess around with my code here: https://repl.it/repls/ConcreteDarkgreenSequence