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 justified interval was returned

But I wonder if I'm relying on undocumented behaviour or if there is any situation where timestamp - timestamp would return a "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