diff --git a/openfisca_core/columns.py b/openfisca_core/columns.py
index <HASH>..<HASH> 100644
--- a/openfisca_core/columns.py
+++ b/openfisca_core/columns.py
@@ -4,6 +4,7 @@
 import collections
 import datetime
 import re
+import warnings
 
 from biryani import strings
 import numpy as np
@@ -221,6 +222,7 @@ class DateCol(Column):
     def __init__(self, default = None, **kwargs):
         super(DateCol, self).__init__(**kwargs)
         if default is None:
+            warnings.warn('DateCol.default not given, using 1970-01-01', DeprecationWarning)
             default = datetime.date.fromtimestamp(0)  # 0 == 1970-01-01
         assert isinstance(default, datetime.date), default
         self.default = default
Add DeprecationWarning with default dates