diff --git a/bin/addons/base/res/res_lang.py b/bin/addons/base/res/res_lang.py
index d19e9389884..f59127318ec 100644
--- a/bin/addons/base/res/res_lang.py
+++ b/bin/addons/base/res/res_lang.py
@@ -19,18 +19,22 @@
 #
 ##############################################################################
 
+import locale
+import logging
+
 from osv import fields, osv
 from locale import localeconv
 import tools
 from tools.safe_eval import safe_eval as eval
 from tools.translate import _
-import locale
-import logging
 
 class lang(osv.osv):
     _name = "res.lang"
     _description = "Languages"
 
+    _disallowed_datetime_patterns = tools.DATETIME_FORMATS_MAP.keys()
+    _disallowed_datetime_patterns.remove('%y') # this one is in fact allowed, just not good practice
+
     def install_lang(self, cr, uid, **args):
         lang = tools.config.get('lang')
         if not lang:
@@ -73,13 +77,14 @@ class lang(osv.osv):
                 return '\xc2\xa0'
             return s
 
-        def fix_time_format(format):
-            """Python's strftime does not support all the compound formats
-               from C's strftime, as returned by locale.nl_langinfo().
-               Under Ubuntu at least, we encounter %T or %r for some locales."""
-            format = format.replace('%T', '%H:%M:%S')\
-                           .replace('%r', '%I:%M:%S %p')\
-                           .replace('%R', '%H:%M')
+        def fix_datetime_format(format):
+            """Python's strftime supports only the format directives
+               that are available on the platform's libc, so in order to
+               be 100% cross-platform we map to the directives required by
+               the C standard (1989 version), always available on platforms
+               with a C standard implementation."""
+            for pattern, replacement in tools.DATETIME_FORMATS_MAP.iteritems():
+                format = format.replace(pattern, replacement)
             return str(format)
 
         lang_info = {
@@ -87,8 +92,8 @@ class lang(osv.osv):
             'iso_code': iso_lang,
             'name': lang_name,
             'translatable': 1,
-            'date_format' : str(locale.nl_langinfo(locale.D_FMT).replace('%y', '%Y')),
-            'time_format' : fix_time_format(locale.nl_langinfo(locale.T_FMT)),
+            'date_format' : fix_datetime_format(locale.nl_langinfo(locale.D_FMT)),
+            'time_format' : fix_datetime_format(locale.nl_langinfo(locale.T_FMT)),
             'decimal_point' : fix_xa0(str(locale.localeconv()['decimal_point'])),
             'thousands_sep' : fix_xa0(str(locale.localeconv()['thousands_sep'])),
         }
@@ -99,6 +104,14 @@ class lang(osv.osv):
             tools.resetlocale()
         return lang_id
 
+    def _check_format(self, cr, uid, ids, context=None):
+        for lang in self.browse(cr, uid, ids, context=context):
+            for pattern in self._disallowed_datetime_patterns:
+                if (lang.time_format and pattern in lang.time_format)\
+                    or (lang.date_format and pattern in lang.date_format):
+                    return False
+        return True
+
     def _get_default_date_format(self,cursor,user,context={}):
         return '%m/%d/%Y'
 
@@ -133,6 +146,10 @@ class lang(osv.osv):
         ('code_uniq', 'unique (code)', 'The code of the language must be unique !'),
     ]
 
+    _constraints = [
+        (_check_format, 'Invalid date/time format directive specified. Please refer to the list of allowed directives, displayed when you edit a language.', ['time_format', 'date_format'])
+    ]
+
     @tools.cache(skiparg=3)
     def _lang_data_get(self, cr, uid, lang_id, monetary=False):
         conv = localeconv()
diff --git a/bin/tools/misc.py b/bin/tools/misc.py
index 015299bcf05..d18090932dc 100644
--- a/bin/tools/misc.py
+++ b/bin/tools/misc.py
@@ -1352,6 +1352,51 @@ DEFAULT_SERVER_DATETIME_FORMAT = "%s %s" % (
     DEFAULT_SERVER_DATE_FORMAT,
     DEFAULT_SERVER_TIME_FORMAT)
 
+# Python's strftime supports only the format directives
+# that are available on the platform's libc, so in order to
+# be cross-platform we map to the directives required by
+# the C standard (1989 version), always available on platforms
+# with a C standard implementation.
+DATETIME_FORMATS_MAP = {
+        '%C': '', # century
+        '%D': '%m/%d/%Y', # modified %y->%Y
+        '%e': '%d',
+        '%E': '', # special modifier
+        '%F': '%Y-%m-%d',
+        '%g': '%Y', # modified %y->%Y
+        '%G': '%Y',
+        '%h': '%b',
+        '%k': '%H',
+        '%l': '%I',
+        '%n': '\n',
+        '%O': '', # special modifier
+        '%P': '%p',
+        '%R': '%H:%M',
+        '%r': '%I:%M:%S %p',
+        '%s': '', #num of seconds since epoch
+        '%T': '%H:%M:%S',
+        '%t': ' ', # tab
+        '%u': ' %w',
+        '%V': '%W',
+        '%y': '%Y', # Even if %y works, it's ambiguous, so we should use %Y
+        '%+': '%Y-%m-%d %H:%M:%S',
+
+        # %Z is a special case that causes 2 problems at least:
+        #  - the timezone names we use (in res_user.context_tz) come
+        #    from pytz, but not all these names are recognized by
+        #    strptime(), so we cannot convert in both directions
+        #    when such a timezone is selected and %Z is in the format
+        #  - %Z is replaced by an empty string in strftime() when
+        #    there is not tzinfo in a datetime value (e.g when the user
+        #    did not pick a context_tz). The resulting string does not
+        #    parse back if the format requires %Z.
+        # As a consequence, we strip it completely from format strings.
+        # The user can always have a look at the context_tz in
+        # preferences to check the timezone.
+        '%z': '',
+        '%Z': '',
+}
+
 def server_to_local_timestamp(src_tstamp_str, src_format, dst_format, dst_tz_name,
         tz_offset=True, ignore_unparsable_time=True):
     """