diff --git a/product/Formulator/Widget.py b/product/Formulator/Widget.py
index 9dc4475a9e32d1927e3564d0d4eb98ac24cd9324..cddc9adbc71a0576470c5020af52065df0605f8a 100644
--- a/product/Formulator/Widget.py
+++ b/product/Formulator/Widget.py
@@ -1533,7 +1533,16 @@ class DateTimeWidget(Widget):
     if not value and field.get_value('default_now'):
       value = DateTime()
     text_node = Element('{%s}%s' % (TEXT_URI, local_name), nsmap=NSMAP)
-    attr_dict['{%s}date-value' % OFFICE_URI] = value.strftime('%Y-%m-%d %H:%M:%S')
+    # http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/#dateTime
+    attr_dict['{%s}date-value' % OFFICE_URI] = value.ISO8601()
+    # According http://wiki.services.openoffice.org/wiki/Documentation/How_Tos/Calc:_Date_%26_Time_functions
+    # default offset is 30/12/1899
+    number_of_second_in_day = 86400 #24 * 60 * 60
+    timestamp = float(value)
+    # XXX Works only if the timezone is the same in OpenOffice
+    ooo_offset_timestamp = float(DateTime(1899, 12, 30))
+    days_value = (timestamp - ooo_offset_timestamp) / number_of_second_in_day
+    attr_dict['{%s}formula' % TEXT_URI] = 'ooow:%f' % days_value
     text_node.attrib.update(attr_dict)
     if as_string:
       return etree.tostring(text_node)