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).

Annex/MetaData/StandardFields.hs

@@ -0,0 +1,38 @@
+{- git-annex metadata, standard fields
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.MetaData.StandardFields (
+	tagMetaField,
+	yearMetaField,
+	monthMetaField,
+	lastChangedField,
+	isLastChangedField
+) where
+
+import Types.MetaData
+
+import Data.List
+
+tagMetaField :: MetaField
+tagMetaField = mkMetaFieldUnchecked "tag"
+
+yearMetaField :: MetaField
+yearMetaField = mkMetaFieldUnchecked "year"
+
+monthMetaField :: MetaField
+monthMetaField = mkMetaFieldUnchecked "month"
+
+lastChangedField :: MetaField -> MetaField
+lastChangedField f = mkMetaFieldUnchecked (fromMetaField f ++ lastchanged)
+
+isLastChangedField :: MetaField -> Bool
+isLastChangedField f = lastchanged `isSuffixOf` s && s /= lastchanged
+  where
+	s = fromMetaField f
+
+lastchanged :: String
+lastchanged = "-lastchanged"