diff --git a/jsonpb/jsonpb.go b/jsonpb/jsonpb.go
index 110ae13842..dfdfc5b300 100644
--- a/jsonpb/jsonpb.go
+++ b/jsonpb/jsonpb.go
@@ -193,8 +193,7 @@ func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent, typeU
 			// "Generated output always contains 3, 6, or 9 fractional digits,
 			//  depending on required precision."
 			s, ns := s.Field(0).Int(), s.Field(1).Int()
-			d := time.Duration(s)*time.Second + time.Duration(ns)*time.Nanosecond
-			x := fmt.Sprintf("%.9f", d.Seconds())
+			x := fmt.Sprintf("%d.%09d", s, ns)
 			x = strings.TrimSuffix(x, "000")
 			x = strings.TrimSuffix(x, "000")
 			out.write(`"`)
diff --git a/jsonpb/jsonpb_test.go b/jsonpb/jsonpb_test.go
index 2428d0566c..4fdbde15a2 100644
--- a/jsonpb/jsonpb_test.go
+++ b/jsonpb/jsonpb_test.go
@@ -407,6 +407,7 @@ var marshalingTests = []struct {
 	{"Any with WKT", marshaler, anyWellKnown, anyWellKnownJSON},
 	{"Any with WKT and indent", marshalerAllOptions, anyWellKnown, anyWellKnownPrettyJSON},
 	{"Duration", marshaler, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 3}}, `{"dur":"3.000s"}`},
+	{"Duration", marshaler, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 100000000, Nanos: 1}}, `{"dur":"100000000.000000001s"}`},
 	{"Struct", marshaler, &pb.KnownTypes{St: &stpb.Struct{
 		Fields: map[string]*stpb.Value{
 			"one": {Kind: &stpb.Value_StringValue{"loneliest number"}},