code: public static Date getDayStartDate(Date date)
     {
        if (date != null)
        {
           Calendar cal = popCalendar(date);
           cal.set(Calendar.HOUR_OF_DAY, 0);
           cal.set(Calendar.MINUTE, 0);
           cal.set(Calendar.SECOND, 0);
           cal.set(Calendar.MILLISECOND, 0);
           date = cal.getTime();
           pushCalendar(cal);
        }
        return (date);
     }
docs: Returns a new Date instance whose value
  represents the start of the day (i.e. the time of day is 00:00:00.000)
  
  @param date date to convert
  @return day start date