On 4/26/2017 1:53 PM, John R Pierce wrote:
On 4/26/2017 1:30 PM, Pietro Pugni wrote:
Adding 10 years to 1912-02-29 returns 1922-02-29, as expected.
I would like to apply the reverse operation. To do so, I subtract 10 years from 1922-02-29 but I obtain 1912-02-28, so the math is actually wrong.
assuming 1922 was a leap year, 1912 is NOT a leap year, so therefore there is no 1912-02-29, that is an invalid date.
ok, I got that backwards. 1912 is the leap year.
date arithmetic is not guaranteed to be associative or commutative due to the irregular units involved.
pierce=# SELECT date '1912-02-29';
date
------------
1912-02-29
(1 row)
pierce=# SELECT date '1922-02-29';
ERROR: date/time field value out of range: "1922-02-29"
LINE 1: SELECT date '1922-02-29';
^
pierce=# SELECT date '1912-02-29' - interval '10 years';
?column?
---------------------
1902-02-28 00:00:00
(1 row)
pierce=# SELECT date '1912-02-29' + interval '10 years';
?column?
---------------------
1922-02-28 00:00:00
(1 row)
as an even more extreme case...
pierce=# SELECT date '2017-04-30' - interval '2 months';
?column?
---------------------
2017-02-28 00:00:00
(1 row)
--
john r pierce, recycling bits in santa cruz