Each for each metadata field, there's now an automatically maintained "$field-lastchanged" that gives the timestamp of the last change to that field.

Note that this is a nearly entirely free feature. The data was already
stored in the metadata log in an easily accessible way, and already was
parsed to a time when parsing the log. The generation of the metadata
fields may even be done lazily, although probably not entirely (the map
has to be evaulated to when queried).

doc/metadata.mdwn

@@ -23,12 +23,15 @@ The field names are limited to alphanumerics (and `[_-.]`), and are case
 insensitive. The metadata values can contain absolutely anything you
 like -- but you're recommended to keep it simple and reasonably short.
 
-Here are some recommended metadata fields to use:
+Here are some metadata fields that git-annex has special support for:
 
 * `tag` - With each tag being a different value.
 * `year`, `month` - When this particular version of the file came into
   being.
-  
+* `$field-lastchanged` - This is automatically maintained for each
+  field that's set, and gives the time stamp (since the Unix epoch)
+  of the most recent change to the field. It cannot be modified directly.
+
 To make git-annex automatically set the year and month when adding files,
 run `git config annex.genmetadata true`. Also, see
 [[tips/automatically_adding_metadata]].

doc/todo/wishlist:_metadata_metadata_view.mdwn

@@ -18,3 +18,6 @@ Something along the lines of
 This would allow me to review files that haven't had any tag changes applied for a while and thus, may need the tags updating.
 
 I've done this in every tagging system I've used by (ab)using mtime, but that requires an additional step (of touching the file).
+
+> [[done]]; "$field-lastchanged" is automatically made available for each
+> field! --[[Joey]]