Golang: How to test functions that use time.Now()? [duplicate] - unit-testing

Question

This question already has answers here:
Is there an easy way to stub out time.Now() globally during test?
(10 answers)
Golang testing programs that involves time
(1 answer)
Closed 21 days ago.
I have a function that is calculating the number of days since a particular timestamp, where the timestamp is coming from an external API (parsed as string in json return from API)
I have been following this article on how to test functions that use time.Now():
https://medium.com/go-for-punks/how-to-test-functions-that-use-time-now-ea4f2453d430
My function looks like this:
type funcTimeType func() time.Time // per suggested in article
func ageOfReportDays(dateString string, funcTime funcTimeType) {
// date string will look like this:
//"2022-08-30 09:05:27.567995"
parseLayout := "2006-01-02 15:04:05.000000"
t, err := time.Parse(parseLayout, dateString)
if err != nil {
fmt.Printf("Error parsing datetime value %v: %w", timeStr, err)
}
days := int(time.Since(t).Abs().Hours() / 24)
//fmt.Println(days)
return days, nil
}
As you can see, I am not using the funcTime funcTimeType in my actual function, as indicated in the article, because I cannot figure out how my function would be implemented with that.
The unit test I would hope to run would be something like this:
func Test_ageOfReportDays(t *testing.T) {
t.Run("timestamp age in days test", func(t *testing.T) {
parseLayout := "2006-01-02 15:04:05.000000"
dateString := "2022-08-30 09:05:27.567995" // example of recent timestamp
mockNow := func() time.Time {
fakeTime, _ := time.Parse(parseLayout, "2023-01-20 09:00:00.000000")
return fakeTime
}
// now I want to use "fakeTime" to spoof "time.Now()" so I can test my function
got: ageOfReportDays(dateString, mockNow)
expected: 152
if got != expected {
t.Errorf("expected '%d' but got '%d'", expected, got)
}
}
Obviously the logic is not quite with my code vs article author's code.
Is there a good way for me to write a unit test for this funcition, based on how the article is suggesting to mock time.Now()?

Answer 1

You are pretty close. Changing time.Since(t) to funcTime().Sub(t) would probably get you passed the finish line.
From time package docs:
time.Since returns the time elapsed since t. It is shorthand for time.Now().Sub(t).
Example function:
import (
"fmt"
"time"
)
const parseLayout = "2006-01-02 15:04:05.000000"
type funcTimeType func() time.Time // per suggested in article
func ageOfReportDays(dateString string, funcTime funcTimeType) (int, error) {
t, err := time.Parse(parseLayout, dateString)
if err != nil {
return 0, fmt.Errorf("parsing datetime value %v: %w", dateString, err)
}
days := int(funcTime().Sub(t).Hours() / 24)
//fmt.Println(days)
return days, nil
}
And a test:
import (
"testing"
"time"
)
func Test_ageOfReportDays(t *testing.T) {
t.Run("timestamp age in days test", func(t *testing.T) {
dateString := "2022-08-30 09:05:27.567995" // example of recent timestamp
mockNow := func() time.Time {
fakeTime, _ := time.Parse(parseLayout, "2023-01-20 09:00:00.000000")
return fakeTime
}
// now I want to use "fakeTime" to spoof "time.Now()" so I can test my function
got, _ := ageOfReportDays(dateString, mockNow)
expected := 142
if got != expected {
t.Errorf("expected '%d' but got '%d'", expected, got)
}
})
}