InfluxDB group by month, the last record is not showing today

Hi All,

I am trying to aggregate my data every 30 days, knowing that InfluxDB cannot group by month, so i used group by time(30d), but it turn out the last record in the query result, the date is not today, i would expect it is showing records every 30 days start from today to the previous days. Any one know how the group by time() calculate the start / end time of the period.