code: public static Calendar blastTime(final Calendar cal) {
          cal.set(Calendar.HOUR_OF_DAY, 0);
          cal.set(Calendar.MINUTE, 0);
          cal.set(Calendar.SECOND, 0);
          cal.set(Calendar.MILLISECOND, 0);
          return cal;
      }
docs: Removes set's all "time" fields to zero, leaving only the date portion of
  the Calendar. The Calendar passe
  
  @param cal
  to Calendar object to blast, note, it will be modified
  @return the calendar object modified (same instance)