001    package org.hackystat.utilities.time.period;
002    
003    
004    /**
005     * Tag interface for <code>Day</code>, <code>Week</code>, <code>Month</code> objects.
006     * 
007     * @author (Cedric) Qin Zhang
008     */
009    public interface TimePeriod extends Comparable<Object> {
010      
011      /**
012       * Returns the first day in this TimePeriod. 
013       * @return The first day associated with this time period. 
014       */
015      public Day getFirstDay();
016    }