LDEV-4092: Date passed back by server was reconverting time for Daylight Saving, making it an hour out, as the convertToStringForJSON takes the timezone into account. Therefore convert the already timezone converted time (which gets converted back again) rather than the original time.
LDEV-4092: Support the server supplying a localised date string for the submission deadline (datetimepicker does not have localisation of the format, just the language).