code: public static TumblingEventTimeWindows of(Time size, Time offset) {
  		return new TumblingEventTimeWindows(size.toMilliseconds(), offset.toMilliseconds());
  	}
docs: Creates a new {@code TumblingEventTimeWindows} {@link WindowAssigner} that assigns
  elements to time windows based on the element timestamp and offset.
  
  <p>For example, if you want window a stream by hour,but window begins at the 15th minutes
  of each hour, you can use {@code of(Time.hours(1),Time.minutes(15))},then you will get
  time windows start at 0:15:00,1:15:00,2:15:00,etc.
  
  <p>Rather than that,if you are living in somewhere which is not using UTC±00:00 time,
  such as China which is using UTC+08:00,and you want a time window with size of one day,
  and window begins at every 00:00:00 of local time,you may use {@code of(Time.days(1),Time.hours(-8))}.
  The parameter of offset is {@code Time.hours(-8))} since UTC+08:00 is 8 hours earlier than UTC time.
  
  @param size The size of the generated windows.
  @param offset The offset which window start would be shifted by.
  @return The time policy.