diff --git a/mercurial/help.py b/mercurial/help.py
--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -435,13 +435,19 @@ PYTHONPATH
     :nonempty:   Any text. Returns '(none)' if the string is empty.
     :hgdate:     Date. Returns the date as a pair of numbers:
                  "1157407993 25200" (Unix timestamp, timezone offset).
-    :isodate:    Date. Returns the date in ISO 8601 format.
+    :isodate:    Date. Returns the date in ISO 8601 format:
+                 "2009-08-18 13:00 +0200".
+    :isodatesec: Date. Returns the date in ISO 8601 format, including
+                 seconds: "2009-08-18 13:00:13 +0200". See also the
+                 rfc3339date filter.
     :localdate:  Date. Converts a date to local date.
     :obfuscate:  Any text. Returns the input text rendered as a
                  sequence of XML entities.
     :person:     Any text. Returns the text before an email address.
     :rfc822date: Date. Returns a date using the same format used in
-                 email headers.
+                 email headers: "Tue, 18 Aug 2009 13:00:13 +0200".
+    :rfc3339date: Date. Returns a date using the Internet date format
+                  specified in RFC 3339: "2009-08-18T13:00:13+02:00".
     :short:      Changeset hash. Returns the short form of a changeset
                  hash, i.e. a 12-byte hexadecimal string.
     :shortdate:  Date. Returns a date like "2006-09-18".