code: @NullSafe
    public static Calendar truncate(Calendar dateTime) {
      Optional.ofNullable(dateTime).ifPresent(localDateTime -> {
        localDateTime.clear(Calendar.HOUR_OF_DAY);
        localDateTime.clear(Calendar.MINUTE);
        localDateTime.clear(Calendar.SECOND);
        localDateTime.clear(Calendar.MILLISECOND);
      });
  
      return dateTime;
    }
docs: Truncates the time portion of the Calendar's date/time components.  If the Calendar object is null, then null
  is returned.  The fields of the Calendar to be truncated include the hour of day, minute, second and milliseconds.
  
  @param dateTime the Calendar object to truncate.
  @return the Calendar object with the time portion truncated (the corresponding time fields of this Calendar
  are set to zero).
  @see java.util.Calendar