date + interval year - why is the return type convered to a timestamp?

Why is Postgres returning a timestamp instead of the expected date data type
for the first expression (the second returns a date)?

In other words, is this a known bug or is it design intent. The manuals did
not seem to appear to say this is intentional behaviour but is not SQL
standard.

select date '2001-03-30' - interval '1' year, date '2001-03-30' - integer
'365' from tversion