Clock is a small library for mocking time in Go. It provides an interface around the standard library's time package so that the application can use the realtime clock while tests can use the mock clock.

Your application can maintain a Clock variable that will allow realtime and mock clocks to be interchangeable. For example, if you had an Application type:

import "github.com/benbjohnson/clock"
type Application struct {
	Clock clock.Clock
}

You could initialize it to use the realtime clock like this:

var app Application
app.Clock = clock.New()
...

Then all timers and time-related functionality should be performed from the Clock variable.

In your tests, you will want to use a Mock clock:

import (
	"testing"
	"github.com/benbjohnson/clock"
)
func TestApplication_DoSomething(t *testing.T) {
	mock := clock.NewMock()
	app := Application{Clock: mock}
	...
}

Now that you've initialized your application to use the mock clock, you can adjust the time programmatically. The mock clock always starts from the Unix epoch (midnight UTC on Jan 1, 1970).

The mock clock provides the same functions that the standard library's time package provides. For example, to find the current time, you use the Now() function:

mock := clock.NewMock()
// Find the current time.
mock.Now().UTC() // 1970-01-01 00:00:00 +0000 UTC
// Move the clock forward.
mock.Add(2 * time.Hour)
// Check the time again. It's 2 hours later!
mock.Now().UTC() // 1970-01-01 02:00:00 +0000 UTC

Timers and Tickers are also controlled by this same mock clock. They will only execute when the clock is moved forward:

mock := clock.NewMock()
count := 0
// Kick off a timer to increment every 1 mock second.
go func() {
    ticker := clock.Ticker(1 * time.Second)
    for {
        <-ticker.C
        count++
    }
}()
runtime.Gosched()
// Move the clock forward 10 seconds.
mock.Add(10 * time.Second)
// This prints 10.
fmt.Println(count)