base: reimplement date formatting for invoice printout

Do not rely on DateTime.strftime which does fails on dates <= 1900.
This simple case does not need all the features of strftime and can be
implemented inline.

bt5/erp5_base/SkinTemplateItem/portal_skins/erp5_base/Delivery_getODTDataDict.py

@@ -63,13 +63,15 @@ def getSocialCapital(reg_cap):
 
 preferred_date_order = context.getPortalObject().portal_preferences\
                                           .getPreferredDateOrder() or 'ymd'
-separator = '/'
 def getOrderedDate(date):
   if date is None:
     return ''
-  pattern = separator.join(['%%%s' % s for s in list(preferred_date_order)])
-  pattern = pattern.replace('y', 'Y')
-  return date.strftime(pattern)
+  date_parts = {
+    'y': '%04d' % date.year(),
+    'm': '%02d' % date.month(),
+    'd': '%02d' % date.day(),
+  }
+  return '/'.join([date_parts[part] for part in preferred_date_order])
 
 def getPaymentConditionText(order):
   if 'custom' == order.getPaymentConditionTradeDate():