Is there an easy way to stub out time.Now() globally during test?

With implementing a custom interface you are already on the right way. I take it you use the following advise from the golang nuts thread you've posted:

---

type Clock interface {
  Now() time.Time
  After(d time.Duration) <-chan time.Time
}

and provide a concrete implementation

type realClock struct{}
func (realClock) Now() time.Time { return time.Now() }
func (realClock) After(d time.Duration) <-chan time.Time { return time.After(d) }

and a testing implementation.

---

^Original

Changing the system time while making tests (or in general) is a bad idea. You don't know what depends on the system time while executing tests and you don't want to find out the hard way by spending days of debugging into that. Just don't do it.

There is also no way to shadow the time package globally and doing that would not do anything more you couldn't do with the interface solution. You can write your own time package which uses the standard library and provides a function to switch to a mock time library for testing if it is the time object you need to pass around with the interface solution that is bothering you.

The best way to design and test your code would probably be to make as much code stateless as possible. Split your functionality in testable, stateless parts. Testing these components separately is much easier then. Also, less side effects means that it is much easier to make the code run concurrently.

I use the bouk/monkey package to replace the time.Now() calls in my code with a fake:

package main

import (
    "fmt"
    "time"

    "github.com/bouk/monkey"
)

func main() {
    wayback := time.Date(1974, time.May, 19, 1, 2, 3, 4, time.UTC)
    patch := monkey.Patch(time.Now, func() time.Time { return wayback })
    defer patch.Unpatch()
    fmt.Printf("It is now %s\n", time.Now())
}

This works well in tests to fake out system dependencies and avoids the abused DI pattern. Production code stays separate from test code and you gain useful control of system dependencies.

If the methods you need to mock are few, such as Now(), you can make a package variable which can be overwritten by tests:

package foo

import "time"

var now = time.Now

// The rest of your code...which calls now() instead of time.Now()

then in your test file:

package foo

import (
    "testing"
    "time"
)

var now = func() time.Time { return ... }

// Your tests