diff --git a/boards/abstracts/sync_filters.py b/boards/abstracts/sync_filters.py
--- a/boards/abstracts/sync_filters.py
+++ b/boards/abstracts/sync_filters.py
@@ -62,5 +62,5 @@ class TimestampFromFilter(PostFilter):
 
     def add_filter(self, model_tag, value):
         tags_from_time = et.SubElement(model_tag, TAG_TIME_FROM)
-        tags_from_time.text = value
+        tags_from_time.text = str(value)
 
diff --git a/boards/management/commands/sync_with_server.py b/boards/management/commands/sync_with_server.py
--- a/boards/management/commands/sync_with_server.py
+++ b/boards/management/commands/sync_with_server.py
@@ -72,6 +72,9 @@ class Command(BaseCommand):
             timestamp = None
             if timestamp_str:
                 timestamp = parse_datetime(timestamp_str)
+                if not timestamp:
+                    raise Exception('Timestamp {} cannot be parsed'.format(
+                        timestamp_str))
 
             xml = SyncManager.generate_request_list(
                 opening_post=options.get('thread'), tags=tags,