diff --git a/README.md b/README.md
index 30598d7..0f1afd4 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,12 @@ Both run in `us-east-1` on the .NET 8 Amazon Linux 2023 platform. Terraform in
 `terraform/live/` owns the environments; GitHub Actions owns the application
 versions. There is no production environment yet.
 
+Stored created, modified and deletion times are UTC: the API stamps them with
+`DateTime.UtcNow`, whatever the host's time zone. The API has only ever run on
+Linux Elastic Beanstalk hosts left at their UTC default (nothing in Terraform,
+`.ebextensions` or `.platform` sets a time zone), so rows written before the
+switch from `DateTime.Now` are already UTC and need no backfill.
+
 ## Architecture
 
 ```text