puzzled time with aggregateWindow() function

Raw data are inserted every 30s. so when the interval param, every, is set in minutes, every interval should have value, the time of which .But that's not.

- When every is setted to "7m", data is retrieved from 00:03:59, which should be "00:08:59"

- When every is setted to "8m", data is retrieved from 00:07:59

Start by reproducing aggregateWindow() with data inserted every 30 seconds and interval values of 7m and 8m. Compare the returned timestamps with the issue's expected boundaries, then trace the aggregateWindow() implementation to identify the alignment behavior. Done means the intervals return the expected timestamps and regression coverage verifies both cases.

Written by the indexing model from the issue text.