TimePeriod.java

public enum TimePeriod {
  ONE_YEAR_AGO(Calendar.YEAR, -1),
  THREE_MONTHS_AGO(Calendar.MONTH, -3), 
  // and so on...
  ;
  
  private int calendarField;
  private int changeMagnitude;
  
  private TimePeriod(int calendarField, int changeMagnitude) {
    Calendar.getInstance().add(calendarField, changeMagnitude).getTime(); // Sanity check
    this.calendarField = calendarField;
    this.changeMagnitude = changeMagnitude;
  }
  
  public long fromNowMillis() {
    return Calendar.getInstance().add(calendarField, changeMagnitude);
  }
  
}