Currently, MessageFormat /only/ supports SimpleDateFormat and instances of java.util.Date or java.util.Calendar for date/time values. Because of this, it's impossible to use Java 8 date/time types with any of the i18n and localization tools.
MessageFormat really needs support for types in java.time. Should I file a bug somewhere about this (where?), or is this message enough to get this going? I'm not even sure how it should work. I'm not sure how MessageFormat can know whether a SimpleDateFormat string or a DateTimeFormatter string is being supplied. Possibly, validation of the format string must be delayed until either a Date/Calendar or a java.time type is supplied for formatting.
