diff -r 80ce81208b08 -r 3051b847c124 server/python/django2/renkanmanager/serializers.py
--- a/server/python/django2/renkanmanager/serializers.py	Tue Jun 21 10:30:08 2016 +0200
+++ b/server/python/django2/renkanmanager/serializers.py	Tue Jun 21 17:53:06 2016 +0200
@@ -3,8 +3,8 @@
 from django.db import transaction
 from django.contrib.auth import get_user_model
 from django.conf import settings
+from django.utils import six
 from django.core.exceptions import ValidationError
-from django.utils import dateparse
 from renkanmanager import settings as renkan_settings
 from renkanmanager.models import Renkan, Workspace, Revision
 from rest_framework import serializers
@@ -23,11 +23,14 @@
     title = serializers.CharField(required=False)
     content = serializers.JSONField(required=False)
     creation_date = serializers.ReadOnlyField()
-    modification_date = serializers.SerializerMethodField("get_current_revision_modification_date")
-    create_new_revision = serializers.BooleanField(write_only=True, required=False) # only used for updating
-    validation_timestamp = serializers.CharField(write_only=True, required=False) # only used for updating is a content json is not provided
+    modification_date = serializers.SerializerMethodField()
+
+    def __init__(self, instance=None, data=serializers.empty, **kwargs):
 
-    # ADD ERROR HANDLING
+        self.validation_timestamp = kwargs.pop('validation_timestamp', None)
+        super(RenkanSerializer, self).__init__(instance, data, **kwargs)
+
+
     def get_current_revision_last_updator(self, renkan):
         last_updator = renkan.current_revision.last_updated_by
         if last_updator:
@@ -36,7 +39,7 @@
             return ''
 
 
-    def get_current_revision_modification_date(self, renkan):
+    def get_modification_date(self, renkan):
         return renkan.current_revision.modification_date
 
     @transaction.atomic
@@ -71,19 +74,12 @@
         content = validated_data.get('content', '')
         if not content:
             content = renkan.current_revision.content
-            if not 'validation_timestamp' in validated_data:
-                raise serializers.ValidationError("No validation timestamp and no content json to extract it from")
-            validation_timestamp = validated_data.get('validation_timestamp')
-        else:
-            validation_timestamp = json.loads(content).get("updated", "")
-        if dateparse.parse_datetime(validation_timestamp) < renkan.current_revision.modification_date:
-            raise serializers.ValidationError("Provided timestamp is invalid")
         if title != json.loads(content).get("title", ""):
             content_dict = json.loads(content)
             content_dict["title"] = title
             content = json.dumps(content_dict)
         try:
-            renkan.save_renkan(updator=updator, timestamp=validation_timestamp, title=title, content=content, create_new_revision=create_new_revision)
+            renkan.save_renkan(updator=updator, timestamp=self.validation_timestamp, title=title, content=content, create_new_revision=create_new_revision)
         except ValidationError as ve:
             raise serializers.ValidationError(str(ve.args[0]))
         # FORCE Renkan reload.
@@ -113,12 +109,16 @@
             raise serializers.ValidationError("Content requires a 'views' entry")
         return value
 
-    def validate_validation_timestamp(self, value):
-        logger.debug("%r < %r", dateparse.parse_datetime(value).timestamp(), self.get_current_revision_modification_date(self.instance).timestamp())
+    def validate(self, data):
+        if self.instance:
+            validation_timestamp = self.validation_timestamp or data.get('content', {}).get('updated')
+            if not validation_timestamp:
+                raise serializers.ValidationError("No validation timestamp and no content json to extract it from")
+            validation_timestamp = dateparse.parse_datetime(validation_timestamp) if isinstance(validation_timestamp, six.string_types) else validation_timestamp
+            if validation_timestamp != self.instance.current_revision.modification_date:
+                raise serializers.ValidationError("Invalid timestamp was provided")
+        return data
 
-        if self.instance and dateparse.parse_datetime(value) != self.get_current_revision_modification_date(self.instance):
-            raise serializers.ValidationError("Invalid timestamp was provided")
-        return value
 
 class RevisionSerializer(serializers.Serializer):
     id = serializers.ReadOnlyField(source="revision_guid")