diff --git a/addons/caldav/caldav_node.py b/addons/caldav/caldav_node.py
index 510e43eabd0..52da451b567 100644
--- a/addons/caldav/caldav_node.py
+++ b/addons/caldav/caldav_node.py
@@ -21,33 +21,14 @@
 
 import time
 from document_webdav import nodes
+from document.nodes import _str2time
 import logging
 import StringIO
 
+from tools.dict_tools import dict_merge, dict_merge2
+
 # TODO: implement DAV-aware errors, inherit from IOError
 
-def dict_merge(*dicts):
-    """ Return a dict with all values of dicts
-    """
-    res = {}
-    for d in dicts:
-        res.update(d)
-    return res
-
-def dict_merge2(*dicts):
-    """ Return a dict with all values of dicts.
-        If some key appears twice and contains iterable objects, the values
-        are merged (instead of overwritten).
-    """
-    res = {}
-    for d in dicts:
-        for k in d.keys():
-            if k in res and isinstance(res[k], (list, tuple)):
-                res[k] = res[k] + d[k]
-            else:
-                res[k] = d[k]
-    return res
-
 # Assuming that we have set global properties right, we mark *all* 
 # directories as having calendar-access.
 nodes.node_dir.http_options = dict_merge2(nodes.node_dir.http_options,
@@ -186,10 +167,18 @@ class node_calendar(nodes.node_class):
         self.content_length = 0
         self.displayname = calendar.name
         self.cal_type = calendar.type
+        # TODO owner
+        
 
     def _get_dav_getctag(self, cr):
-        result = self._get_ttag(cr) + ':' + str(time.time())
-        return str(result)
+        dirobj = self.context._dirobj
+        uid = self.context.uid
+        ctx = self.context.context.copy()
+        ctx.update(self.dctx)
+
+        bc_obj = dirobj.pool.get('basic.calendar')
+        res = bc_obj.get_cal_max_modified(cr, uid, [self.calendar_id], self, domain=[], context=ctx)
+        return _str2time(res)
 
     def _get_dav_user_state(self, cr):
         #TODO
@@ -295,11 +284,9 @@ class node_calendar(nodes.node_class):
         if not domain:
             domain = []
 
-        fil_obj = dirobj.pool.get('basic.calendar')
-        ids = fil_obj.search(cr, uid, domain)
-        res = []
-        if self.calendar_id in ids:
-            res = fil_obj.get_calendar_objects(cr, uid, [self.calendar_id], self, domain=where, context=ctx)
+        bc_obj = dirobj.pool.get('basic.calendar')
+        # we /could/ be supplying an invalid calendar id to bc_obj, it has to check
+        res = bc_obj.get_calendar_objects(cr, uid, [self.calendar_id], self, domain=where, context=ctx)
         return res
 
     def create_child(self, cr, path, data):
diff --git a/addons/caldav/calendar.py b/addons/caldav/calendar.py
index 2f7d978b5a6..2133e3be293 100644
--- a/addons/caldav/calendar.py
+++ b/addons/caldav/calendar.py
@@ -552,6 +552,33 @@ class CalDAV(object):
             self.ical_reset('value')
         return res
 
+if True: # we need this indentation level ;)
+
+    def get_last_modified(self, cr, user, args, context=None, access_rights_uid=None):
+        """Return the last modification date of objects in 'domain'
+        This function has similar semantics to orm.search(), apart from the
+        limit, offset and order arguments, which make no sense here.
+        It is useful when we want to find if the table (aka set of records)
+        has any modifications we should update at the client.
+        """
+        if context is None:
+            context = {}
+        self.pool.get('ir.model.access').check(cr, access_rights_uid or user, self._name, 'read', context=context)
+
+        query = self._where_calc(cr, user, args, context=context)
+        self._apply_ir_rules(cr, user, query, 'read', context=context)
+        from_clause, where_clause, where_clause_params = query.get_sql()
+
+        where_str = where_clause and (" WHERE %s" % where_clause) or ''
+
+        cr.execute('SELECT MAX(COALESCE("%s".write_date, "%s".create_date)) FROM ' % (self._table, self._table) + 
+                    from_clause + where_str ,
+                    where_clause_params,
+                    debug=self._debug)
+        res = cr.fetchall()
+        return res[0][0]
+
+
 class Calendar(CalDAV, osv.osv):
     _name = 'basic.calendar'
     _calname = 'calendar'
@@ -610,6 +637,32 @@ class Calendar(CalDAV, osv.osv):
                     node = res_node_calendar('%s.ics' %data.id, parent, ctx, data, line.object_id.model, data.id)
                     res.append(node)
         return res
+        
+
+    def get_cal_max_modified(self, cr, uid, ids, parent=None, domain=None, context=None):
+        if not context:
+            context = {}
+        if not domain:
+            domain = []
+        res = None
+        ctx_res_id = context.get('res_id', None)
+        ctx_model = context.get('model', None)
+        for cal in self.browse(cr, uid, ids):
+            for line in cal.line_ids:
+                if ctx_model and ctx_model != line.object_id.model:
+                    continue
+                if line.name in ('valarm', 'attendee'):
+                    continue
+                line_domain = eval(line.domain or '[]', context)
+                line_domain += domain
+                if ctx_res_id:
+                    line_domain += [('id','=',ctx_res_id)]
+                mod_obj = self.pool.get(line.object_id.model)
+                max_data = get_last_modified(mod_obj, cr, uid, line_domain, context=context)
+                if res and res > max_data:
+                    continue
+                res = max_data
+        return res
 
     def export_cal(self, cr, uid, ids, vobj='vevent', context=None):
         """ Export Calendar