FastDateFormatter.java

import java.nio.charset.Charset;
import java.text.SimpleDateFormat;
import java.util.Arrays;
import java.util.Date;
import java.util.TimeZone;

public class FastDateFormatter
{
    private static final byte[][] DATES = new byte[24837][];    // days in [1970, 2038)
    private static final byte[] DIGITS = toUtf8("0123456789");
    private static final long MS_PER_DAY = 24L * 60L * 60L * 1000L;
    private static final long MS_PER_HOUR = 60L * 60L * 1000L;
    private static final long MS_PER_MINUTE = 60L * 1000L;

    static {
        for (int i = 0; i < DATES.length; i++) {
            long t = i * MS_PER_DAY;
            DATES[i] = toUtf8(formatDateSlow(t).substring(0, 10));
            assert formatDateSlow(t).substring(11).equals("00:00:00.000");
        }
    }

    @SuppressWarnings({"NumericCastThatLosesPrecision"})
    public static byte[] formatDate(long t)
    {
        long save = t;

        // days from epoch start
        long days = t / MS_PER_DAY;

        // fallback if outside pre-computed range
        if ((t < 0) || (days >= DATES.length)) {
            return toUtf8(formatDateSlow(t));
        }

        // remove days
        t %= MS_PER_DAY;

        // extract time
        int hour = (int) (t / MS_PER_HOUR);
        t %= MS_PER_HOUR;
        int min = (int) (t / MS_PER_MINUTE);
        t %= MS_PER_MINUTE;
        int sec = (int) (t / 1000L);
        int msec = (int) (t % 1000L);
        assert (hour < 24) && (min < 60) && (sec < 60) && (msec < 1000);

        // format date and time (YYYY-MM-DD hh:mm:ss.SSS)
        byte[] b = new byte[23];
        System.arraycopy(DATES[((int) days)], 0, b, 0, 10);
        b[10] = (byte) ' ';
        b[11] = DIGITS[hour / 10];
        b[12] = DIGITS[hour % 10];
        b[13] = (byte) ':';
        b[14] = DIGITS[min / 10];
        b[15] = DIGITS[min % 10];
        b[16] = (byte) ':';
        b[17] = DIGITS[sec / 10];
        b[18] = DIGITS[sec % 10];
        b[19] = (byte) '.';
        b[20] = DIGITS[msec / 100];
        b[21] = DIGITS[(msec % 100) / 10];
        b[22] = DIGITS[msec % 10];

        assert Arrays.equals(b, toUtf8(formatDateSlow(save)));

        return b;
    }

    private static String formatDateSlow(long t)
    {
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
        format.setTimeZone(TimeZone.getTimeZone("UTC"));
        return format.format(new Date(t));
    }

    private static byte[] toUtf8(String s)
    {
        return s.getBytes(Charset.forName("UTF-8"));
    }

    public static void main(String[] args)
    {
        System.out.println(new String(formatDate(1269465111345L), Charset.forName("UTF-8")));
    }
}