Package clock implements a library for mocking time. Include a Clock variable on your application and initialize it with a Realtime() by default. Then use the Clock for all time-related API calls. So instead of time.NewTimer(), say myClock.NewTimer(). On a test setup, override or inject the variable with a Mock instance and use it to control how the time behaves during each test phase. To mock context.WithTimeout and context.WithDeadline, use the included Context, TimeoutContext and DeadlineContext methods. The Context method is also useful in cases where you need to pass a Clock via an 'func(ctx Context, ..)' API you can't change yourself. The FromContext method will then return the associated Clock instance. Alternatively, use the context'ed methods like Sleep(ctx) directly. All methods are safe for concurrent use.