PostgreSQL interval subtraction - postgresql

Question

I've tried to subtract interval from timestamp, but I've got a wrong result in comparison to days via subtracting 2 dates.
E.g.:
select
(now::date - past::date) as days,
(now::date - past::date) / 365.25 as years,
justify_interval(now - past::date) as interval_test
from (
select '2020-09-17 00:00:01'::timestamp as now, '2010-09-17 00:00:01'::timestamp as past
) b;
gives results:
3653 days
10.0013 years
'10 years 1 mon 23 days' interval test
Could anyone help me to understand what is wrong with subtracting?
When I do it vice versa, it's ok:
select
(past::date + 3653)::date,
(past + interval '10 years')::date,
(past + 10*interval '1 year')::date,
(past + 10*12*interval '1 month')::date
from (
select '2020-09-17 00:00:01'::timestamp as now, '2010-09-17 00:00:01'::timestamp as past
) b;
all results give the same date '2020-09-17'
What I do wrong?
I am using PostgreSQL 10.5.

Answer 1

There is nothing wrong with subtracting. It is just that justify_interval doesn't do what you seem to expect. justify_interval uses 30 day months and 24 hour days. So 12 months becomes only 360 and 10 years only 3600 days. Leaving 53 days which is 1 (30 day) month and 23 days.
Edit
The justify_interval documentation on this page refers to justify_days and justify_hours which are directly above it which do mention the use of 30 days months and 24 hour days.
The justify functions do have to make these assumption because the interval type is a general length of time (it has no specific start and end). So the justify functions does not know over which specific months the interval was originally calculated.
The age function however does not take an interval it takes an end and a start so it actually knows which specific months and years are in that period.