diff -r dcf80f5b7b63 -r c2230b7f6358 cubicweb/web/formwidgets.py
--- a/cubicweb/web/formwidgets.py	Tue Mar 07 11:59:07 2017 +0100
+++ b/cubicweb/web/formwidgets.py	Tue Mar 07 12:08:39 2017 +0100
@@ -795,17 +795,17 @@
         req = form._cw
         datestr = req.form.get(field.input_name(form, 'date'))
         if datestr:
-            datestr = datestr.strip() or None
-        timestr = req.form.get(field.input_name(form, 'time'))
-        if timestr:
-            timestr = timestr.strip() or None
-        if datestr is None:
+            datestr = datestr.strip()
+        if not datestr:
             return None
         try:
             date = todatetime(req.parse_datetime(datestr, 'Date'))
         except ValueError as exc:
             raise ProcessFormError(text_type(exc))
-        if timestr is None:
+        timestr = req.form.get(field.input_name(form, 'time'))
+        if timestr:
+            timestr = timestr.strip()
+        if not timestr:
             return date
         try:
             time = req.parse_datetime(timestr, 'Time')