diff --git a/tstime/mono/mono.go b/tstime/mono/mono.go
index 6b02bb41a..4042c53a5 100644
--- a/tstime/mono/mono.go
+++ b/tstime/mono/mono.go
@@ -104,7 +104,8 @@ func (t Time) WallTime() time.Time {
 
 // MarshalJSON formats t for JSON as if it were a time.Time.
 // We format Time this way for backwards-compatibility.
-// This is best-effort only. Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged.
+// Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged
+// across different invocations of the Go process. This is best-effort only.
 // Since t is a monotonic time, it can vary from the actual wall clock by arbitrary amounts.
 // Even in the best of circumstances, it may vary by a few milliseconds.
 func (t Time) MarshalJSON() ([]byte, error) {
@@ -113,7 +114,8 @@ func (t Time) MarshalJSON() ([]byte, error) {
 }
 
 // UnmarshalJSON sets t according to data.
-// This is best-effort only. Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged.
+// Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged
+// across different invocations of the Go process. This is best-effort only.
 func (t *Time) UnmarshalJSON(data []byte) error {
 	var tt time.Time
 	err := tt.UnmarshalJSON(data)
@@ -124,6 +126,6 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 		*t = 0
 		return nil
 	}
-	*t = Now().Add(-time.Since(tt))
+	*t = baseMono.Add(tt.Sub(baseWall))
 	return nil
 }
diff --git a/tstime/mono/mono_test.go b/tstime/mono/mono_test.go
index a78f49791..f425abe9a 100644
--- a/tstime/mono/mono_test.go
+++ b/tstime/mono/mono_test.go
@@ -33,6 +33,21 @@ func TestUnmarshalZero(t *testing.T) {
 	}
 }
 
+func TestJSONRoundtrip(t *testing.T) {
+	want := Now()
+	b, err := want.MarshalJSON()
+	if err != nil {
+		t.Errorf("MarshalJSON error: %v", err)
+	}
+	var got Time
+	if err := got.UnmarshalJSON(b); err != nil {
+		t.Errorf("UnmarshalJSON error: %v", err)
+	}
+	if got != want {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
+
 func BenchmarkMonoNow(b *testing.B) {
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {