Constants defined in an interface not visible to implementing subclass in a static context

public interface DateTimeFormatConstants {
    static final SimpleDateFormat AM_PM_TIME_FORMAT = new SimpleDateFormat("h:mma")

    static final SimpleDateFormat MILITARY_TIME_FORMAT = new SimpleDateFormat("HH:mm")
}

class DateTimeUtils implements DateTimeFormatConstants {
    static String convertMilitaryTimeToAmPm(String militaryTime) {
        Date date = MILITARY_TIME_FORMAT.parse(militaryTime)
        return AM_PM_TIME_FORMAT.format(date).toLowerCase()
    }
}

You attempted to reference a variable in the binding or an instance variable from a static context.
You misspelled a classname or statically imported field. Please check the spelling.
You attempted to use a method 'AM_PM_TIME_FORMAT' but left out brackets in a place not allowed by the grammar.
 @ line 13, column 16.
           return AM_PM_TIME_FORMAT.format(date).toLowerCase()
                  ^