Auto-advance

By default, MockTimeSystem advances its clock every time the code under test waits. A call to Thread.Sleep(5s) or Task.Delay(5s) returns immediately and the next read of DateTime.Now reflects 5 seconds later. This is the right default - most tests want to skip the wait, not enforce it.

MockTimeSystem timeSystem = new(new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc));

await timeSystem.Task.Delay(TimeSpan.FromMinutes(10));
timeSystem.DateTime.UtcNow; // 2026-01-01 00:10:00 UTC

Disabling auto-advance

When you need to assert that code waited (instead of just letting it through), turn auto-advance off:

MockTimeSystem timeSystem = new(o => o.DisableAutoAdvance());

await timeSystem.Task.Delay(TimeSpan.FromMinutes(10));
// Clock did NOT move - Delay returned because nothing simulates progress

With auto-advance off you control time yourself by writing into the time provider. Combine this with the notifications hooks to assert what your code asked for.

Multi-threaded scenarios

The default time provider stores the clock in a single field, so two parallel Sleep(5s) calls move it by 10 seconds total, which can be surprising. If your test exercises real parallelism, swap in the thread-aware provider so each virtual thread has its own clock.