Flink has this option to accept an optional offset parameter that can be used to change the alignment of windows.

Example: without offsets hourly tumbling windows are aligned with epoch, that is you will get windows such as 1:00:00.000 - 1:59:59.999, 2:00:00.000 - 2:59:59.999 and so on. If you want to change that you can give an offset. With an offset of 15 minutes you would, for example, get 1:15:00.000 - 2:14:59.999, 2:15:00.000 - 3:14:59.999 etc.

An important use case for offsets is to adjust windows to timezones other than UTC-0. For example, in China you would have to specify an offset of Time.hours(-8).