synctest: fix comments for time.Now() in synctests

Also add a test case to make sure that time.Now() results in the
documented date.

Change-Id: Ic4cc577eba485b7c6e1a64122da06d7075bbe12e
Reviewed-on: https://go-review.googlesource.com/c/go/+/685677
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Christian Höppner <hoeppi@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

Author: hoeppi-google

src/testing/synctest/example_test.go

@@ -21,7 +21,7 @@ import (
 
 func TestTime(t *testing.T) {
 	synctest.Test(t, func(t *testing.T) {
-		start := time.Now() // always midnight UTC 2001-01-01
+		start := time.Now() // always midnight UTC 2000-01-01
 		go func() {
 			time.Sleep(1 * time.Nanosecond)
 			t.Log(time.Since(start)) // always logs "1ns"

src/testing/synctest/synctest.go

@@ -22,7 +22,7 @@
 //
 //	func TestTime(t *testing.T) {
 //		synctest.Test(t, func(t *testing.T) {
-//			start := time.Now() // always midnight UTC 2001-01-01
+//			start := time.Now() // always midnight UTC 2000-01-01
 //			go func() {
 //				time.Sleep(1 * time.Second)
 //				t.Log(time.Since(start)) // always logs "1s"

src/testing/synctest/synctest_test.go

@@ -11,6 +11,7 @@ import (
 	"regexp"
 	"testing"
 	"testing/synctest"
+	"time"
 )
 
 // Tests for interactions between synctest bubbles and the testing package.
@@ -179,3 +180,11 @@ func runTest(t *testing.T, args []string, f func(), pattern string) {
 		t.Errorf("got output:\n%s\nwant matching:\n%s", out, pattern)
 	}
 }
+
+func TestNow(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		if got, want := time.Now(), time.Date(2000, time.January, 1, 0, 0, 0, 0, time.UTC); !got.Equal(want) {
+			t.Errorf("time.Now() = %v, want %v", got, want)
+		}
+	})
+}