A Date which is stable regarding hashCode, equals and compareTo

StableDate.java

import java.io.Serializable;
import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

/**
 * A UTC date, consisting of a year, a month and a day component. Compared to {@linkplain java.util.Date}
 * a StableDate is stable regarding {@linkplain java.lang.Object#equals} and {@linkplain java.lang.Object#hashCode}.
 */
public class StableDate implements Comparable<StableDate>, Serializable {

    private static final long serialVersionUID = 1L;

    private final int year;
    private final int month;
    private final int day;

    /**
     * Creates a stable date based on a time in UTC milliseconds.
     * @param millis the time in UTC milliseconds from the epoch.
     * @return A new StableDate instance
     */
    public static StableDate of(long millis) {
        // java util date/time stuff is considered to be not thread safe - therefore creating new instances here
        final Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
        calendar.setTimeInMillis(millis);
        final int year = calendar.get(Calendar.YEAR);
        final int month = calendar.get(Calendar.MONTH) + 1;
        final int day = calendar.get(Calendar.DAY_OF_MONTH);
        return new StableDate(year, month, day);
    }

    /**
     * Creates a stable date based on a UTC {@linkplain java.util.Date}.
     * @param date A Date
     * @return A new StableDate instance
     */
    public static StableDate of(Date date) {
        return StableDate.of(date.getTime());
    }

    /**
     * Constructs a StableDate.
     * @param year A year.
     * @param month A month, starting with 1.
     * @param day A day, starting with 1.
     */
    StableDate(int year, int month, int day) {
        this.year = year;
        this.month = month;
        this.day = day;
    }

    /**
     * Gets this year.
     * @return this year
     */
    public int getYear() {
        return year;
    }

    /**
     * Gets this month.
     * @return this month
     */
    public int getMonth() {
        return month;
    }

    /**
     * Gets this day.
     * @return this day
     */
    public int getDay() {
        return day;
    }

    /**
     * Compares this and that StableDate.
     * @param that A StableDate, may be null
     * @return A negativ number, 0 or a positive number if this date is before, equal or after that date. If that is null, 1 is returned.
     */
    @Override
    public int compareTo(StableDate that) {
        return (that == null) ? 1 : (this.year - that.year) * 10000 + (this.month - that.month) * 100 + (this.day - that.day);
    }

    /**
     * Checks, if o equals this. An object o is equal to this, if o is an instance of StableDate and
     * the year, month and day fields of this and o are respectively the same.
     * @param o An object, may be null.
     * @return true, if o equals this, false otherwise
     */
    @Override
    public boolean equals(Object o) {
        if (o == this) {
            return true;
        } else if (o instanceof StableDate) {
            final StableDate that = (StableDate) o;
            return this.year == that.year && this.month == that.month && this.day == that.day;
        } else {
            return false;
        }
    }

    /**
     * Calculates the hash of this StableDate.
     * @return {@code year * 10000 + month * 100 + day}
     */
    @Override
    public int hashCode() {
        return year * 10000 + month * 100 + day;
    }

    /**
     * Returns a String representation of this StableDate according to ISO 8601 format 'YYYY-MM-DD'.
     * @return This date as String
     */
    @Override
    public String toString() {
        return String.format("%04d-%02d-%02d", year, month, day);
    }
}