diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 13c0ddc7c..953668106 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -754,7 +754,7 @@ uploads_path: "DATADIR/uploads"
 # then derive an expiration date from the current validity period), and some time
 # after that the validity period changes and Synapse is restarted, the users'
 # expiration dates won't be updated unless their account is manually renewed. This
-# date will be randomly selected within a range [now + period ; now + period + d],
+# date will be randomly selected within a range [now + period - d ; now + period],
 # where d is equal to 10% of the validity period.
 #
 #account_validity: