the issue you're running into is a lack of events because they dropped out of your DB. default event table retention is only 30 days by default.
if you want to change this setting, you can; but it can cause some negative consequences re: your platform health because of the sheer amount of events that are generally created. there's a decent guide available that covers this topic.
for your reference, you were on the right track!
DATEADD( <interval>, <(-) range>, <fromDateTime>)
from your example:
eventtime between dateadd(month, -1, getdate()) and getdate()
this is saying:
WHERE the 'EventTime' data from the table is between -1 month from today and today
- getdate(), is a function that returns the datetime from the moment of query execution
So, when you changed "-1" to "-6", you effectively said, between 6 months ago and today.
That being said, because datetime() returns "now"; you could just as easily use a clause like this:
EventTime > DATEADD( month, -6, GETDATE() )
so; kudos on the effort!