"lookup relationship can to span multiple layer deep" - What it means?

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;
}
}

I need test class for this class.Please provide me

Apex class as following-

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) {
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;
}
}
-------------------------------------------------
@Test class as following-

@isTest
public class TestVerifyDate
{
     @isTest static void testWarmTemp()
     {
          Date dte = VerifyDate.CheckDates(1,10);
          System.assertEquals(10,dte);
     }
}
--------------------------

test code show me the following error-
[Error] Error: Compile Error: Method does not exist or incorrect signature: VerifyDate.CheckDates(Integer, Integer) at line 6 column 21