Very simple datetime for Java (minimal port from C#)

DateTime.java

public class DateTime implements Cloneable, Comparable<DateTime>,Parcelable {
    public static DateTime today() { return now().withTimeAtStartOfDay(); }
    public static DateTime now() { return new DateTime(Calendar.getInstance()); }

    private final Calendar calendar;

    @Override
    public int compareTo(@NonNull DateTime another) {
        return calendar.compareTo(another.calendar);
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return new DateTime((Calendar) calendar.clone());
    }

    public DateTime(int year, int month, int dayOfMonth) {
        this.calendar = new GregorianCalendar(year, month, dayOfMonth);
    }

    public DateTime(int year, int month, int dayOfMonth, int hour, int minute, int seconds) {
        this.calendar = new GregorianCalendar(year, month, dayOfMonth, hour, minute, seconds);
    }

    public DateTime(Calendar calendar) {
        if(calendar == null) throw new IllegalArgumentException("Calendar cannot be null");
        this.calendar = calendar;
    }

    public DateTime(Date date) {
        if(date == null) throw new IllegalArgumentException("Date cannot be null");
        this.calendar = new GregorianCalendar();
        this.calendar.setTime(date);
    }

    private DateTime newDateTime(CalendarChanger calendarChanger) {
        Calendar clone = (Calendar) calendar.clone();
        calendarChanger.change(clone);
        return new DateTime(clone);
    }

    /**
     * @return the day of the month, ranging between 1 - 31 (depending on the month)
     */
    public int getDayOfMonth() {
        return calendar.get(Calendar.DAY_OF_MONTH);
    }

    /**
     * See Calendar.MONDAY, Calendar.TUESDAY, ...
     * @return the day of the week, ranging between 1 - 7 where 1 is Sunday and 7 is Saturday.
     */
    public int getDayOfWeek() {
        return calendar.get(Calendar.DAY_OF_WEEK);
    }

    /**
     * @return the day of the year, ranging between 1 and 365
     */
    public int getDayOfYear() {
        return calendar.get(Calendar.DAY_OF_YEAR);
    }

    /**
     * See Calendar.JANUARY, Calendar.FEBRUARY, ...
     * @return the month, ranging between 0 and 11 where 0 is January and 11 is December.
     */
    public int getMonth() {
        return calendar.get(Calendar.MONTH);
    }

    /**
     * @return the year
     */
    public int getYear() {
        return calendar.get(Calendar.YEAR);
    }

    /**
     * @param days the number of days to add (can be negative)
     * @return a new DateTime offset with the provided days
     */
    public DateTime addDays(final int days) {
        return newDateTime(new CalendarChanger() {
            @Override
            public void change(Calendar calendar) {
                calendar.add(Calendar.DATE, days);
            }
        });
    }

    /**
     * @param months the number of months to add (can be negative)
     * @return a new DateTime offset with the provided months
     */
    public DateTime addMonths(final int months) {
        return newDateTime(new CalendarChanger() {
            @Override
            public void change(Calendar calendar) {
                calendar.add(Calendar.MONTH, months);
            }
        });
    }

    /**
     * @param years the number of years to add (can be negative)
     * @return a new DateTime offset with the provided years
     */
    public DateTime addYears(final int years) {
        return newDateTime(new CalendarChanger() {
            @Override
            public void change(Calendar calendar) {
                calendar.add(Calendar.YEAR, years);
            }
        });
    }

    /**
     * @return a new DateTime with all the time fields set to zero (essentially turning back the clock to midnight)
     */
    public DateTime withTimeAtStartOfDay() {
        return newDateTime(new CalendarChanger() {
            @Override
            public void change(Calendar calendar) {
                calendar.set(Calendar.HOUR_OF_DAY, 0);
                calendar.set(Calendar.MINUTE, 0);
                calendar.set(Calendar.SECOND, 0);
                calendar.set(Calendar.MILLISECOND, 0);
            }
        });
    }

    /**
     * @param other the other DateTime
     * @return true if this instance is even just a millisecond before the other datetime or false otherwise
     */
    public boolean isBefore(DateTime other) {
        return this.calendar.compareTo(other.calendar) < 0;
    }

    /**
     * @param other the other DateTime
     * @return true if this instance is even just a millisecond after the other datetime or false otherwise
     */
    public boolean isAfter(DateTime other) {
        return this.calendar.compareTo(other.calendar) > 0;
    }

    /**
     * @param other the other DateTime
     * @return true if this instance has the exact same date fields (year, month, day) as the other datetime or false otherwise
     */
    public boolean isSameDayAs(DateTime other) {
        return this.getYear() == other.getYear()
            && this.getDayOfYear() == other.getDayOfYear();
    }

    /**
     * @return a new Calendar representing the same date as this DateTime
     */
    public Calendar toCalendar() {
        return (Calendar) calendar.clone();
    }

    /**
     * @return a new Date representing the same date as this DateTime
     */
    public Date toDate() {
        return calendar.getTime();
    }

    public long toTimeInMilliseconds() { return calendar.getTimeInMillis(); }

    @Override
    public String toString() {
        return "DateTime{" +
                DateTimeFormat.ISO_8601.format(this) +
                '}';
    }

    private interface CalendarChanger {
        public void change(Calendar calendar);
    }

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeSerializable(this.calendar);
    }

    private DateTime(Parcel in) {
        this.calendar = (Calendar) in.readSerializable();
    }

    public static final Parcelable.Creator<DateTime> CREATOR = new Parcelable.Creator<DateTime>() {
        public DateTime createFromParcel(Parcel source) {
            return new DateTime(source);
        }

        public DateTime[] newArray(int size) {
            return new DateTime[size];
        }
    };
}

DateTimeFormat.java

public class DateTimeFormat {
    public static final DateTimeFormat VERY_LONG = new DateTimeFormat("EEEE d MMMM yyyy");
    public static final DateTimeFormat ISO_8601 = new DateTimeFormat("yyyy-MM-dd'T'HH:mm'Z'");

    public static DateTimeFormat getTimeFormat(Context context) {
        return new DateTimeFormat(android.text.format.DateFormat.getTimeFormat(context));
    }

    private final DateFormat dateFormat;

    public DateTimeFormat(DateFormat dateFormat) {
        if(dateFormat == null) {
            throw new IllegalArgumentException("Cannot create a DateTimeFormat without a format specified: dateFormat cannot be null");
        }
        this.dateFormat = dateFormat;
    }

    /**
     * Creates a new DateTimeFormat with the given format string and the specified locale
     * @param format the format to be used, see http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html for options
     */
    public DateTimeFormat(String format, Locale locale) {
        this(new SimpleDateFormat(format, locale));
    }

    /**
     * Creates a new DateTimeFormat with the given format string and the user preferred locale
     * @param format the format to be used, see http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html for options
     */
    public DateTimeFormat(String format) {
        this(format, Locale.getDefault());
    }

    public String format(DateTime dateTime) {
        return dateFormat.format(dateTime.toDate());
    }

    public DateTime parse(String formattedDateTime) throws ParseException {
        Date date = dateFormat.parse(formattedDateTime);
        Calendar calendar = new GregorianCalendar();
        calendar.setTime(date);
        return new DateTime(calendar);
    }
}