diff --git a/addons/project/project.py b/addons/project/project.py
index 12b1569f025..b91c61d5f21 100644
--- a/addons/project/project.py
+++ b/addons/project/project.py
@@ -1048,7 +1048,7 @@ class task(osv.osv):
         if vals.get('project_id') and not context.get('default_project_id'):
             context['default_project_id'] = vals.get('project_id')
         # user_id change: update date_start
-        if vals.get('user_id'):
+        if vals.get('user_id') and not vals.get('start_date'):
             vals['date_start'] = fields.datetime.now()
 
         # context: no_log, because subtype already handle this
@@ -1065,7 +1065,7 @@ class task(osv.osv):
         if 'stage_id' in vals:
             vals['date_last_stage_update'] = fields.datetime.now()
         # user_id change: update date_start
-        if vals.get('user_id'):
+        if vals.get('user_id') and 'date_start' not in vals:
             vals['date_start'] = fields.datetime.now()
 
         # Overridden to reset the kanban_state to normal whenever