diff --git a/SftpFrontend.md b/SftpFrontend.md
index eee005c..f3560b1 100644
--- a/SftpFrontend.md
+++ b/SftpFrontend.md
@@ -11,6 +11,7 @@ Since Tahoe uses capability access control rather than Unix-style permissions, t
 
 See the last section of [FTP-and-SFTP.txt]source:docs/frontends/FTP-and-SFTP.txt for information on how the SFTP frontend treats immutable and mutable files.
 
+The '`ctime`' and '`mtime`' fields will not be updated when a mutable file is changed, and do not have exactly the semantics specified by POSIX in other cases.
 
 ### sshfs