[FIX] web: use correct timezone for date fields

before this commit when toggling allday flag in calendar form view, it was
possible to have one day error.

The input datetime is wrong when the user don't use the datepicker widget
 (insert date by keypress instead of click on datepicker)

sometimes, the value could be null (crash because try to clone false)

addons/web/static/tests/fields/basic_fields_tests.js

@@ -1041,7 +1041,10 @@ QUnit.module('basic_fields', {
                   '</tree>',
             translateParameters: {  // Avoid issues due to localization formats
                 date_format: '%m/%d/%Y',
-            }
+            },
+            session: {
+                tzOffset: 0
+            },
         });
 
         var $cell = list.$('tr.o_data_row td:not(.o_list_record_selector)').first();