Time-Based Key-Value Store

Implement a time-based key-value store that supports expiring keys. Your data structure should support the following operations:

1. set(key, value, ttl): Store the key with its associated value and a time-to-live (ttl) in seconds. The key should be considered expired and inaccessible after the ttl has passed.

2. get(key): Retrieve the value associated with the key if it exists and has not expired. If the key does not exist or has expired, return null.

Additional requirements:

• The operations should be efficient, aiming for O(1) average time complexity for both set and get.
• Consider how you might clean up expired keys either on-the-fly or via a background process to manage memory usage.
• You may assume access to a function that returns the current time in seconds.

Design and implement this data structure in the programming language of your choice.