diff --git a/app/nullius/catalogue/capabilities/mem0/capability.py b/app/nullius/catalogue/capabilities/mem0/capability.py
index afd1e5d..7e4c44c 100644
--- a/app/nullius/catalogue/capabilities/mem0/capability.py
+++ b/app/nullius/catalogue/capabilities/mem0/capability.py
@@ -12,7 +12,8 @@
 
 ``add {session}``     - the session's turns as Mem0 messages (a role other than
                         user or assistant is a user turn naming its speaker),
-                        added for one user, the session's id and date as metadata
+                        added for one user, the session's id and date as metadata,
+                        the date also as ``timestamp``
 ``recall {question}`` - ``{memory}``: what Mem0's search returns, best first,
                         each memory headed by its session's date
 
@@ -111,8 +112,10 @@ def add(self, s):
         self.dates[sid] = str(s.get("date") or "")
         self.warnings.take()
         t0 = time.time()
-        got = self.mem.add(messages, user_id=USER, metadata={"session": sid, "date": self.dates[sid]},
-                           infer=setting("MEM0_INFER", "on") != "off")
+        meta = {"session": sid, "date": self.dates[sid]}
+        if self.dates[sid]:
+            meta["timestamp"] = self.dates[sid]
+        got = self.mem.add(messages, user_id=USER, metadata=meta, infer=setting("MEM0_INFER", "on") != "off")
         items = got.get("results") if isinstance(got, dict) else got
         return {"trace": {"messages": len(messages), "seconds": round(time.time() - t0, 3),
                           "written": [{k: it.get(k) for k in ("id", "event", "memory", "previous_memory") if it.get(k) is not None}
diff --git a/app/tests/test_mem0_capability.py b/app/tests/test_mem0_capability.py
new file mode 100644
index 0000000..2ed54a7
--- /dev/null
+++ b/app/tests/test_mem0_capability.py
@@ -0,0 +1,53 @@
+"""The mem0 capability's add: what it hands Mem0, with a stand-in for Mem0."""
+import importlib.util
+import os
+import sys
+import types
+import unittest
+
+HERE = os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))),
+                    "nullius", "catalogue", "capabilities", "mem0", "capability.py")
+
+
+class FakeMem0:
+    def __init__(self):
+        self.added = []
+
+    def add(self, messages, **kw):
+        self.added.append(kw)
+        return {"results": []}
+
+    def get_all(self, **kw):
+        return {"results": []}
+
+
+def load():
+    out = sys.stdout                       # the module sends stdout to stderr when it loads
+    try:
+        spec = importlib.util.spec_from_file_location("mem0_capability_under_test", HERE)
+        mod = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(mod)
+    finally:
+        sys.stdout = out
+    return mod
+
+
+class Add(unittest.TestCase):
+    def setUp(self):
+        self.mod = load()
+        self.m = self.mod.Memory()
+        self.m.mem = FakeMem0()
+
+    def test_the_session_date_is_also_the_timestamp_mem0_reads(self):
+        self.m.add({"id": "s1", "date": "2023/05/21 (Sun) 16:22", "turns": [{"role": "user", "text": "hi"}]})
+        self.assertEqual(self.m.mem.added[0]["metadata"],
+                         {"session": "s1", "date": "2023/05/21 (Sun) 16:22", "timestamp": "2023/05/21 (Sun) 16:22"})
+
+    def test_an_undated_session_has_no_timestamp(self):
+        # an empty timestamp would be an Observation Date of nothing; without one Mem0 keeps its default
+        self.m.add({"id": "s1", "turns": [{"role": "user", "text": "hi"}]})
+        self.assertNotIn("timestamp", self.m.mem.added[0]["metadata"])
+
+
+if __name__ == "__main__":
+    unittest.main()