diff --git a/demo/.gitignore b/demo/.gitignore
index 3f68360..74df5b7 100644
--- a/demo/.gitignore
+++ b/demo/.gitignore
@@ -1 +1 @@
-demo.db
+demo.kantadb
diff --git a/demo/main.py b/demo/main.py
index 05b0491..ba3efe6 100644
--- a/demo/main.py
+++ b/demo/main.py
@@ -3,12 +3,14 @@
 Run from the project root:  python demo/main.py
 
 Demonstrates bootstrap, colored transaction diffs, logfmt value formatting,
-custom log headers, logging toggles, rollback, and migrations.  The database
-is recreated on every run; everything else lives in this file.
+custom log headers, logging toggles, rollback, migrations, and a custom clock.
+The database is recreated with fixed timestamps on every run; everything else
+lives in this file.
 """
 
 import asyncio
 import logging
+from datetime import UTC, datetime, timedelta
 from pathlib import Path
 from types import ModuleType
 
@@ -18,12 +20,25 @@ from kanta import Kanta
 from kanta.logging import _ACTION, _ACTOR, _RESET, _SESSION, _TARGET
 from kanta.logging import configure_logging
 
-DB = Path(__file__).with_name("demo.db")
+DB = Path(__file__).with_name("demo.kantadb")
 
 # Fake directory: user id -> display name, resolved by the logfmt callbacks.
 USERS = {"u1": "Alice", "u2": "Bob", "u3": "Carol"}
 
 
+class Clock:
+    """Deterministic clock: manually advanced, so every run is identical."""
+
+    def __init__(self) -> None:
+        self.current = datetime(2026, 8, 6, 12, 0, tzinfo=UTC)
+
+    def advance(self, **kwargs) -> None:
+        self.current += timedelta(**kwargs)
+
+
+clock = Clock()
+
+
 class DataV1(msgspec.Struct):
     """Original schema (version 0)."""
 
@@ -48,6 +63,14 @@ migrations = ModuleType("demo_migrations")
 migrations.migrate_v1 = migrate_v1
 
 
+def add_clock(kanta: Kanta) -> None:
+    """Use the shared deterministic clock for all record timestamps."""
+
+    @kanta.clock
+    def fake_now() -> datetime:
+        return clock.current
+
+
 def add_logfmts(kanta: Kanta) -> None:
     """Resolve user ids to display names in headers and diff paths."""
 
@@ -73,12 +96,17 @@ def add_header(kanta: Kanta) -> None:
         return f"{actor} {session}  {_ACTION}{action}{_RESET}  {target}"
 
 
+def section(title: str) -> None:
+    print(f"\n# {title}", flush=True)
+
+
 async def main() -> None:
     DB.unlink(missing_ok=True)
 
-    print("=== Standard logging: bootstrap, diffs, toggles, rollback ===", flush=True)
+    section("Standard logging: bootstrap, diffs, toggles, rollback")
 
     kanta = Kanta(DB, DataV1())
+    add_clock(kanta)
     add_logfmts(kanta)
 
     @kanta.bootstrap
@@ -87,23 +115,29 @@ async def main() -> None:
 
     await kanta.open()
 
+    clock.advance(minutes=2)
     with kanta.transaction(action="create", user="u2") as data:
         data.users["u2"] = {"name": "Bob", "role": "user"}
 
+    clock.advance(minutes=5)
     with kanta.transaction(action="update", user="u1") as data:
         data.users["u2"]["role"] = "editor"
         data.counter = 1
 
+    clock.advance(seconds=30)
     with kanta.transaction(action="delete", user="u1") as data:
         del data.users["u2"]
 
     # Display-only extra string, appended after the action.
+    clock.advance(hours=1)
     with kanta.transaction(action="export", user="u1", extra=DB.name) as data:
         data.counter = 2
 
-    # Compact logging: diff only (no header) ...
+    # Compact logging, diff only: a system fix stamped by the clock, but the
+    # modification time (m) is not updated.
+    clock.advance(minutes=10)
     with kanta.transaction(
-        action="repair", log={"header": False, "diff": True}
+        action="repair", mtime=False, log={"header": False, "diff": True}
     ) as data:
         data.users["u3"] = {"name": "Carol", "role": "user"}
 
@@ -115,7 +149,8 @@ async def main() -> None:
     except ValueError:
         pass
 
-    # ... and header only (no diff).
+    # Compact logging, header only.
+    clock.advance(minutes=5)
     with kanta.transaction(
         action="import", user="u1", log={"header": True, "diff": False}
     ) as data:
@@ -123,17 +158,21 @@ async def main() -> None:
 
     await kanta.close()
 
-    print("=== Reopen with migrations and a custom log header ===", flush=True)
+    section("Reopen with migrations and a custom log header")
 
+    clock.advance(days=1)
     kanta = Kanta(DB, Data(), migrations=migrations)
+    add_clock(kanta)
     add_logfmts(kanta)
     add_header(kanta)
     await kanta.open()
 
     # No target given: defaults to the database filename.
+    clock.advance(minutes=3)
     with kanta.transaction(action="update", user="u1", extra={"session_id": 3}) as data:
         data.settings["theme"] = "light"
 
+    clock.advance(minutes=1)
     with kanta.transaction(
         action="update",
         user="u2",
@@ -143,7 +182,9 @@ async def main() -> None:
 
     await kanta.close()
 
-    print(f"=== Database written to {DB} ===", flush=True)
+    # The pretty names only exist in the logs; the database stores raw ids.
+    section("Raw database records (user ids and timestamps, not pretty names)")
+    print(DB.read_text(), end="", flush=True)
 
 
 if __name__ == "__main__":
diff --git a/docs/database.md b/docs/database.md
index f7f09eb..475650e 100644
--- a/docs/database.md
+++ b/docs/database.md
@@ -163,6 +163,15 @@ when they have a default value.
 - Multiple handlers are supported and invoked in registration order.  A failing
   handler is logged and does not prevent subsequent handlers from running.
 
+#### Clock
+
+- `@kanta.clock` registers a callback `() -> datetime` that replaces the
+  default UTC clock.  Its value is used for all record timestamps (`ts`, and
+  `m` when `mtime` is `True`) and for snapshot timestamps.
+- Register before `open()` so that bootstrap and migration records use the
+  custom clock as well.  This is mainly useful for tests and reproducible
+  demos.
+
 #### Transaction Log Formatting
 
 - Logfmt callbacks prettify identifiers in the change log and are registered with