diff --git a/internal/quic/qlog/json_writer.go b/internal/quic/qlog/json_writer.go
index 3950ab42f6..b2fa3e03e5 100644
--- a/internal/quic/qlog/json_writer.go
+++ b/internal/quic/qlog/json_writer.go
@@ -157,6 +157,10 @@ func (w *jsonWriter) writeBoolField(name string, v bool) {
 
 // writeDuration writes a duration as milliseconds.
 func (w *jsonWriter) writeDuration(v time.Duration) {
+	if v < 0 {
+		w.buf.WriteByte('-')
+		v = -v
+	}
 	fmt.Fprintf(&w.buf, "%d.%06d", v.Milliseconds(), v%time.Millisecond)
 }
 
diff --git a/internal/quic/qlog/json_writer_test.go b/internal/quic/qlog/json_writer_test.go
index 7ba5e17378..6da5566412 100644
--- a/internal/quic/qlog/json_writer_test.go
+++ b/internal/quic/qlog/json_writer_test.go
@@ -124,9 +124,10 @@ func TestJSONWriterBoolField(t *testing.T) {
 func TestJSONWriterDurationField(t *testing.T) {
 	w := newTestJSONWriter()
 	w.writeRecordStart()
-	w.writeDurationField("field", (10*time.Millisecond)+(2*time.Nanosecond))
+	w.writeDurationField("field1", (10*time.Millisecond)+(2*time.Nanosecond))
+	w.writeDurationField("field2", -((10 * time.Millisecond) + (2 * time.Nanosecond)))
 	w.writeRecordEnd()
-	wantJSONRecord(t, w, `{"field":10.000002}`)
+	wantJSONRecord(t, w, `{"field1":10.000002,"field2":-10.000002}`)
 }
 
 func TestJSONWriterFloat64Field(t *testing.T) {