diff --git a/doc/changelog.rst b/doc/changelog.rst
index eadfe81b5e8..46f84364c76 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -6,6 +6,8 @@ Changelog
 `trunk`
 -------
 
+- Added support of custom group_by format and display format when using group_by
+  on a datetime field, using datetime_format context key
 - Improved ``html_email_clean`` in tools: better quote and signature finding,
   added shortening.
 - Cleaned and slightly refactored ``ir.actions.server``. The ``loop``, ``sms``
diff --git a/openerp/osv/orm.py b/openerp/osv/orm.py
index f081e11b9e2..25ed8bdf861 100644
--- a/openerp/osv/orm.py
+++ b/openerp/osv/orm.py
@@ -2601,7 +2601,18 @@ class BaseModel(object):
         :param list groupby: fields by which the records will be grouped
         :param int offset: optional number of records to skip
         :param int limit: optional max number of records to return
-        :param dict context: context arguments, like lang, time zone
+        :param dict context: context arguments, like lang, time zone. A special
+                             context key exist for datetime fields : ``datetime_format``.
+                             context[``datetime_format``] = {
+                                'field_name': {
+                                    groupby_format: format for to_char (default: yyyy-mm)
+                                    display_format: format for displaying the value
+                                                    in the result (default: MMM yyyy)
+                                    interval: day, month or year; used for begin
+                                              and end date of group_by intervals
+                                              computation (default: month)
+                                }
+                             }
         :param list orderby: optional ``order by`` specification, for
                              overriding the natural sort ordering of the
                              groups, see also :py:meth:`~osv.osv.osv.search`