code: @CheckForNull
    public static Date parseEndingDateOrDateTime(@Nullable String stringDate) {
      if (stringDate == null) {
        return null;
      }
  
      Date date = parseDateTimeQuietly(stringDate);
      if (date != null) {
        return date;
      }
  
      date = parseDateQuietly(stringDate);
      checkArgument(date != null, "Date '%s' cannot be parsed as either a date or date+time", stringDate);
  
      return addDays(date, 1);
    }
docs: Return the datetime if @param stringDate is a datetime, date + 1 day if stringDate is a date.
  So '2016-09-01' would return a date equivalent to '2016-09-02T00:00:00+0000' in GMT (Warning: relies on default timezone!)
  
  @return the datetime, {@code null} if stringDate is null
  @throws IllegalArgumentException if stringDate is not a correctly formed date or datetime
  @see #parseDateOrDateTime(String)
  @since 6.1