diff --git a/docs/sources/reference/builder.md b/docs/sources/reference/builder.md
index 1f18f0c6..00d9ba3e 100644
--- a/docs/sources/reference/builder.md
+++ b/docs/sources/reference/builder.md
@@ -376,7 +376,11 @@ destination container.
 All new files and directories are created with a UID and GID of 0.
 
 In the case where `<src>` is a remote file URL, the destination will
-have permissions of 600.
+have permissions of 600. If the remote file being retrieved has an HTTP
+`Last-Modified` header, the timestamp from that header will be used
+to set the `mtime` on the destination file. Then, like any other file
+processed during an `ADD`, `mtime` will be included in the determination
+of whether or not the file has changed and the cache should be updated.
 
 > **Note**:
 > If you build by passing a `Dockerfile` through STDIN (`docker