diff --git a/client.go b/client.go
index d059b83..86a4dfe 100644
--- a/client.go
+++ b/client.go
@@ -218,6 +218,7 @@ func ChallengeTs(window time.Duration) Criterion {
 		if diff := now().Sub(r.ChallengeTs); diff > window {
 			return &InvalidChallengeTsError{
 				ChallengeTs: r.ChallengeTs,
+				Diff:        diff,
 			}
 		}
 		return nil
diff --git a/client_test.go b/client_test.go
index e2850ab..f020d30 100644
--- a/client_test.go
+++ b/client_test.go
@@ -325,6 +325,7 @@ func TestVerify(t *testing.T) {
 			},
 			expected: &InvalidChallengeTsError{
 				ChallengeTs: now().Add(-time.Second),
+				Diff:        time.Second,
 			},
 		},
 		{
diff --git a/errors.go b/errors.go
index 53eddcd..16db355 100644
--- a/errors.go
+++ b/errors.go
@@ -58,8 +58,9 @@ func (e *InvalidScoreError) Error() string {
 // window.
 type InvalidChallengeTsError struct {
 	ChallengeTs time.Time
+	Diff        time.Duration
 }
 
 func (e *InvalidChallengeTsError) Error() string {
-	return fmt.Sprintf("invalid reCAPTCHA: invalid challenge timestamp: %s", e.ChallengeTs)
+	return fmt.Sprintf("invalid reCAPTCHA: invalid challenge timestamp: %s (%s old)", e.ChallengeTs, e.Diff)
 }