[GENERAL] Result of timestamp - timestamp

Hello,

I just stumbled about a report that has been running for a long time now and that relied on the fact that the interval
"timestamp- timestamp" always returns an interval with days, hours, minutes. But never a "justified" interval with
years,months, days and so on. 

The query usees "extract(day from timestamp - timestamp)" which is working fine, but would apparently fail if a
justifiedinterval was returned 

But I wonder if I'm relying on undocumented behaviour or if there is any situation where timestamp - timestamp would
returna "justified" interval. 

So, my question is: will timestamp '2017-02-05 18:19:20' - timestamp '2016-11-18 23:00:00' always return "78 days
21:00:00"?

Or is there any situation where the returned interval would be "2 mons 18 days 21:00:00" without using
justiy_interval()on it. 

I couldn't find a clear statement on that in the manual.

Thomas