Apache Pinot

Hi, I have a question about `DATETIMECONVERT` . It mentions it buckets the time based on the given time granularity, but what is the start of the first bucket? e.g. when running this query:
```SELECT DATETIMECONVERT(time_col, '1:MILLISECONDS:EPOCH', '1:SECONDS:EPOCH', '30:DAYS') as new_time_col, COUNT(id) FROM table WHERE (time_col BETWEEN &lt;epoch_second of 7/16&gt; AND &lt;epoch_second of 9/16&gt;) AND GROUP BY new_time_col ORDER BY new_time_col```
It returns 3 buckets for: 7/1, 7/31 and 8/30. So I wonder how is this being calculated?

For a bit of more context, we are trying to bucket our data with size of 30-day bucket to categorize their age.

Same result using
```DATETIMECONVERT(time_col, '1:MILLISECONDS:EPOCH', '1:SECONDS:EPOCH', '720:HOURS')```

The start of the first bucket is Unix epoch time, and we use millis since epoch to calculate the time bucket