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Configuring time

MockTimeSystem gets its current time from an ITimeProvider. Three factories cover most cases:

// 1. Random instant per test (default - useful for catching tests that pass only "today")
new MockTimeSystem();

// 2. Pin to a specific instant (UTC if Kind is Unspecified)
new MockTimeSystem(new DateTime(2026, 1, 15, 10, 0, 0, DateTimeKind.Utc));

// 3. Use the host's current wall clock
new MockTimeSystem(TimeProvider.Now());

TimeProvider.Use(DateTime), TimeProvider.Random() and TimeProvider.Now() all return an ITimeProviderFactory you can pass to the MockTimeSystem constructor.

Custom time providers​

Implement ITimeProvider directly when you need something more interesting than a single DateTime - for example, a clock that ticks per virtual thread. See Thread-aware time provider for a worked example.

Wrap your provider in a factory and pass it in:

ITimeProviderFactory factory = new MyTimeProviderFactory(...);
MockTimeSystem timeSystem = new(factory);

The factory hands the mock an Action<DateTime> callback to invoke whenever the simulated time changes. That callback drives the notifications and the auto-advance behaviour.

Sharing the time system​

Pass an existing MockTimeSystem into a MockFileSystem so file-system timestamps and time-related assertions stay consistent:

MockTimeSystem timeSystem = new(new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc));
MockFileSystem fileSystem = new(o => o.UseTimeSystem(timeSystem));

fileSystem.File.WriteAllText("a.txt", "hi");
DateTime created = fileSystem.File.GetCreationTimeUtc("a.txt");
// created is anchored to 2026-01-01