Test class to control Singleton Stub

SingletonTest.cls

@IsTest
public with sharing class SingletonTest {
    @IsTest
    public static void testLeapYear() {
        //not stubbed out, so this will take the normal path w/o the switch
        SingletonController.printDateTime();
    }

    @IsTest
    public static void testNonLeapYear() {
        //IS stubbed out, so this WILL take the alternate path
        DateTime newNow = DateTime.newInstance(2019, 9, 8);
        DateTimeProvider.instance = new DateTimeStub(newNow);
        SingletonController.printDateTime();
        //in the wild, return data and make assertions here
    }
    
    private with sharing class DateTimeStub extends DateTimeProvider {
        public DateTime now;
        
        public DateTimeStub(DateTime now) {
            this.now = now;
        }
        
        public override DateTime now() {
            return now;
        }
    }

}