diff --git a/evo/tools/tf_cache.py b/evo/tools/tf_cache.py
index 2a0cce6..8a39131 100644
--- a/evo/tools/tf_cache.py
+++ b/evo/tools/tf_cache.py
@@ -24,8 +24,14 @@
 import math
 import warnings
 from collections import defaultdict
-from typing import (DefaultDict, List, Optional, Protocol, Union,
-                    runtime_checkable)
+from typing import (
+    DefaultDict,
+    List,
+    Optional,
+    Protocol,
+    Union,
+    runtime_checkable,
+)
 
 import numpy as np
 import tf2_py
@@ -60,6 +66,9 @@ def to_sec(self) -> float:
         Gets scalar time, in seconds.
         """
 
+    def __lt__(self, other) -> bool:
+        """ less-than comparator, required for sorting """
+
 
 @runtime_checkable
 class Ros2TimeLike(Protocol):  # pylint: disable=too-few-public-methods
@@ -72,6 +81,9 @@ def nanoseconds(self) -> int:
         Gets underlying scalar timestamp, in nanoseconds.
         """
 
+    def __lt__(self, other) -> bool:
+        """ less-than comparator, required for sorting """
+
 
 @runtime_checkable
 class Ros2StampLike(Protocol):  # pylint: disable=too-few-public-methods