diff --git a/master/buildbot/db/build_data.py b/master/buildbot/db/build_data.py
index 5ac33a50d9e..70687a1688a 100644
--- a/master/buildbot/db/build_data.py
+++ b/master/buildbot/db/build_data.py
@@ -167,14 +167,13 @@ def thd(conn) -> int:
                 q = q.where(
                     (builds.c.complete_at >= older_than_timestamp) | (builds.c.complete_at == NULL)
                 )
+                # n.b.: in sqlite we need to filter on `>= older_than_timestamp` because of the following `NOT IN` clause...
 
                 q = build_data.delete().where(build_data.c.buildid.notin_(q))
             else:
                 q = build_data.delete()
                 q = q.where(builds.c.id == build_data.c.buildid)
-                q = q.where(
-                    (builds.c.complete_at >= older_than_timestamp) | (builds.c.complete_at == NULL)
-                )
+                q = q.where(builds.c.complete_at <= older_than_timestamp)
             res = conn.execute(q)
             conn.commit()
             res.close()
diff --git a/newsfragments/janitor-timehorizon.bugfix b/newsfragments/janitor-timehorizon.bugfix
new file mode 100644
index 00000000000..b3e7f21d31d
--- /dev/null
+++ b/newsfragments/janitor-timehorizon.bugfix
@@ -0,0 +1 @@
+Fixes the timestamp comparison in janitor: it should cleanup builds 'older than' given timestamp - previously janitor would delete all build_data for builds 'newer than' the configured build data horizon.
\ No newline at end of file