Design a task scheduler in Python.

A task scheduler can be implemented using a class that manages a list of tasks. Each task would have a description and an optional interval for recurring execution. The scheduler would have a method to add new tasks and a main run loop. This loop continuously checks the current time against each task's scheduled next run time. If a task is due, it's executed, and its next run time is updated (either immediately for one-off tasks or based on its interval for recurring tasks). The loop continues for a specified duration or until explicitly stopped.