Salesforce Error: Invisible Future Method in Test Class?

Hello,

Problem in your test class is that you are try to access the private method in your test class. Which is wrong.
As per your code you need to call only VerifyDate.CheckDates(System.today(),System.today()+1); method which will call your private method in class.

So simply copy paste below test class and try

@isTest  public class TestVerifyDate {
static testMethod void testMethod1()
{
Date d = VerifyDate.CheckDates(System.today(),System.today()+1);         Date d1 = VerifyDate.CheckDates(System.today(),System.today()+60);
}

}