How do you format the day of the month to say “11th”, “21st” or “23rd” (ordinal indicator)?

Question

I know this will give me the day of the month as a number (11, 21, 23):

SimpleDateFormat formatDayOfMonth = new Simple         


        

Answer 1

Here is an approach that updates a DateTimeFormatter pattern with the correct suffix literal if it finds the pattern d'00', e.g. for day of month 1 it would be replaced with d'st'. Once the pattern has been updated it can then just be fed into the DateTimeFormatter to do the rest.

private static String[] suffixes = {"th", "st", "nd", "rd"};

private static String updatePatternWithDayOfMonthSuffix(TemporalAccessor temporal, String pattern) {
    String newPattern = pattern;
    // Check for pattern `d'00'`.
    if (pattern.matches(".*[d]'00'.*")) {
        int dayOfMonth = temporal.get(ChronoField.DAY_OF_MONTH);
        int relevantDigits = dayOfMonth < 30 ? dayOfMonth % 20 : dayOfMonth % 30;
        String suffix = suffixes[relevantDigits <= 3 ? relevantDigits : 0];
        newPattern = pattern.replaceAll("[d]'00'", "d'" + suffix + "'");
    }

    return newPattern;
}

It does require that the original pattern is updated just prior to every formatting call, e.g.

public static String format(TemporalAccessor temporal, String pattern) {
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern(updatePatternWithDayOfMonthSuffix(temporal, pattern));
    return formatter.format(temporal);
}

So this is useful if the formatting pattern is defined outside of Java code, e.g. a template, where as if you can define the pattern in Java then the answer by @OleV.V. might be more appropriate