User Login Tracking

Design a user login tracking system that handles a log of user login actions. Each login is marked with a username and a timestamp. Implement functionality to count the number of unique users that logged in during a certain period defined by a start and end timestamp. Return the count of unique users. Example: Input: 5 user1 2023-10-01T10:00:00 user2 2023-10-01T10:05:00 user1 2023-10-01T10:10:00 user3 2023-10-01T10:15:00 user1 2023-10-01T10:20:00 2023-10-01T10:00:00 2023-10-01T10:15:00 Output: 3