I've found it in caldavprotocol.cpp:34

        QString startTime = QDateTime::currentDateTimeUtc().addMonths(-3).toString("yyyyMMddTHHMMssZ");

I don't think it's a good practice to hardcode such values. Given you have a calendar in a DAV resource and you intend using it for the first time with KOrganizer, you don't have a chance to get all events. So you can't archive older events to another place anymore if you missed that hidden deadline. There should be at least an option to override that limit temporarily.

I could work around it by setting the system date back in the past, but after the date has been refetched from a NTP server, I've lost all older events in the DAV resource again!

Please make that value configurable or allow to toggle that filter condition for those who need to access the filtered data by whatever purpose.

Reproducible: Always

Steps to Reproduce:
1. Set up a calender e.g. in Owncloud and configure the DAV resource to show that calendar in KOrganizer.
2. Create (or import) one or more events at least 3 months in the past in that Owncloud calender (let's assume the data was synced there from another device).
3. Try to see that event(s) in KOrganizer,.

Actual Results:  
You won't be able to see those events in KOrganizer.

Expected Results:  
Korganizer should allow to list all events. That way you can use the archiving function in KOrganizer. Else the events may vanish before you had a chance to backup them in KOrganizer.

It looks like a data loss, although it actually isn't. So I keep the severity normal. However it's a really bad feeling if someone can't look behind the scenes and simply misses events out of a sudden.