testland/dotnet-faketime

Wraps .NET's TimeProvider abstraction (System.TimeProvider, introduced .NET 8) and FakeTimeProvider from Microsoft.Extensions.TimeProvider.Testing: SetUtcNow, Advance, AutoAdvanceAmount, CreateTimer, Delay, and the pre-.NET-8 ISystemClock migration path. Use when testing C# or F# code that reads the current time, uses timers, or awaits Task.Delay.

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Anti-patterns and limitations

Name: testland/dotnet-faketime
Rating: 75.2 (1 reviews)
Author: testland

Anti-patterns

Anti-pattern	Why it fails	Fix
`DateTime.UtcNow` directly in code	Not injectable; test is forced to use wall-clock	Inject `TimeProvider`; call `_time.GetUtcNow()`
`DateTimeOffset.UtcNow` directly in code	Same problem	Inject `TimeProvider`
Static mock of `DateTime` via Fakes/Harmony	Requires special test runner config or IL rewriting	Use DI with `TimeProvider`
Call `SetUtcNow` with a value earlier than current	Throws `ArgumentOutOfRangeException`	Use `Advance` or construct fresh `FakeTimeProvider`
Forget `await Task.Yield()` after `Advance`	Continuations haven't had a chance to run yet	Yield or await the completed task
`AutoAdvanceAmount` in tests needing exact instants	Clock shifts unexpectedly between reads	Set `AutoAdvanceAmount = TimeSpan.Zero` (default)
Register `TimeProvider.System` in test DI	Tests become time-dependent and flaky	Register `FakeTimeProvider` in test DI setup

Limitations

Task.Delay(int) overloads that do NOT accept a TimeProvider still use wall-clock time. Always use the timeProvider.Delay(TimeSpan) extension form.
Thread.Sleep is not controlled by FakeTimeProvider; restructure to use await timeProvider.Delay(...) instead.
High-frequency GetTimestamp() values are derived from the fake UTC instant, not from Stopwatch. Per learn.microsoft.com/dotnet/api/microsoft.extensions.time.testing.faketimeprovider.timestampfrequency, TimestampFrequency is a fixed value tied to the fake clock.
Third-party libraries that call DateTime.UtcNow internally are not affected by FakeTimeProvider; only code that accepts TimeProvider by injection can be controlled this way.
No leap-second simulation; see leap-second-reference.