[SalesForce] Method Is Not Visible: APEX Trailhead Unit Testing Challenge

PROBLEM:
I am receiving a "Method is not visible: Boolean VerifyDate.DateWithin30Days(Date, Date)" on my 3rd, 4th, and 5th. I am also receiving a "Method is not visible: Date VerifyDate.SetEndOfMonthDate(Date)" on my 6th method. Does anyone have any ideas? Your help would be greatly appreciated. Thank you!

CHALLENGE:
Create a unit test for a simple Apex class.

Install a simple Apex class, write unit tests that achieve 100% code coverage for the class, and run your Apex tests.

The Apex class to test is called 'VerifyDate', and the code is available here. Copy and paste this class into your Developer Edition via the Developer Console.
'VerifyDate' is a class which tests if a date is within a proper range, and if not will return a date that occurs at the end of the month within the range.
The unit tests must be in a separate test class called 'TestVerifyDate'.
The unit tests must cover scenarios for all lines of code included in the Apex class, resulting in 100% code coverage.

Run your test class at least once (via 'Run All' tests the Developer Console) before attempting to verify this challenge.

VERIFY DATE:

public class VerifyDate {

    //method to handle potential checks against two dates
    public static Date CheckDates(Date date1, Date date2) {
        //if date2 is within the next 30 days of date1, use date2.  Otherwise use the end of the month
        if(DateWithin30Days(date1,date2)) {
            return date2;
        } else {
            return SetEndOfMonthDate(date1);
        }
    }

    //method to check if date2 is within the next 30 days of date1
    private static Boolean DateWithin30Days(Date date1, Date date2) {
        //check for date2 being in the past
            if( date2 < date1) { return false; }

            //check that date2 is within (>=) 30 days of date1
            Date date30Days = date1.addDays(30); //create a date 30 days away from date1
        if( date2 >= date30Days ) { return false; }
        else { return true; }
    }

    //method to return the end of the month of a given date
    private static Date SetEndOfMonthDate(Date date1) {
        Integer totalDays = Date.daysInMonth(date1.year(), date1.month());
        Date lastDay = Date.newInstance(date1.year(), date1.month(), totalDays);
        return lastDay;
    }

}

MY CODE FOR TESTING:

@isTest
private class TestVerifyDate
    {
      @isTest static void testCheckDatesOne ()
      {
        Date test = VerifyDate.CheckDates (Date.newInstance(2018, 7, 19), Date.newInstance(2018, 7, 20));
        System.assertEquals(Date.newInstance(2018, 7, 20), test);
      }

    @isTest static void testCheckDatesTwo ()
    {
        Date test = VerifyDate.CheckDates (Date.newInstance(2018, 7, 19), Date.newInstance(2018, 8, 20));
        System.assertEquals(Date.newInstance(2018, 8, 20), test);

    }

    @isTest static void testDateWithin30DaysOne ()
    {
        boolean test = VerifyDate.DateWithin30Days (Date.newInstance(2018, 7, 19), Date.newInstance(2018, 7, 18));
        System.assertEquals(false, test);
    }

    @isTest static void testDateWithin30DaysTwo ()
    {
        boolean test = VerifyDate.DateWithin30Days (Date.newInstance(2018, 7, 19), Date.newInstance(2019, 1, 1));
        System.assertEquals(false, test);
    }

    @isTest static void testDateWithin30DaysThree ()
    {
        boolean test = VerifyDate.DateWithin30Days (Date.newInstance(2018, 7, 19), Date.newInstance(2018, 7, 19));
        System.assertEquals(true, test);
    }

    @isTest static void testSetEndOfMonthDate ()
    {
        Date test = VerifyDate.SetEndOfMonthDate (Date.newInstance(2018, 7, 19));
        System.assertEquals(Date.newInstance(2018, 7, 31), test);
    }

}