code: public static boolean isSameInstant(final Calendar cal1, final Calendar cal2) {
          if (cal1 == null || cal2 == null) {
              throw new IllegalArgumentException("The date must not be null");
          }
          return cal1.getTime().getTime() == cal2.getTime().getTime();
      }
docs: <p>Checks if two calendar objects represent the same instant in time.</p>
  
  <p>This method compares the long millisecond time of the two objects.</p>
  
  @param cal1  the first calendar, not altered, not null
  @param cal2  the second calendar, not altered, not null
  @return true if they represent the same millisecond instant
  @throws IllegalArgumentException if either date is <code>null</code>
  @since 2.1