The default implementation of the ITimeProvider in Testably.Abstractions.Testing uses a DateTime property to simulate the current time, that is advanced with every Thread.Sleep or Task.Delay call.
In a scenario with multiple threads running in parallel, these would each influence each other differently in the mocked instance than "in the real world": Two thread running in parallel, each sleeping for 5s should only advance the total clock by 5s, but in the mocked instance they would advance the clock by 10s.
This example illustrates how to implement a thread-aware time provider for such a scenario.
The ThreadAwareTimeProvider stores the time in a AsyncLocal property and provides a SynchronizeClock method to synchronize the clock across multiple threads.
Corresponding unit tests show the correct behaviour.
In order to use this, you just have to inject the custom provider to the MockTimeSystem:
MockTimeSystem timeSystem = new(new ThreadAwareTimeProvider());