Re: [BUGS] BUG #14632: Plus and minus operators inconsistency withleap years and year intervals.

I’ll try to reformulate better.

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.

The “logical” bug can be seen also by adding and subtracting the same quantity:

SELECT '1912-02-29'::date - '10 years'::interval + '10 years'::interval

It returns '1912-02-28 00:00:00' instead of '1912-02-29 00:00:00'.

It’s an issue because there’s no other way to obtain the original date (reversing the add operation).

Also:

SELECT age('1922-02-28'::date, '1912-02-29'::date) = '10 years'::interval

returns false, while:

SELECT ('1922-02-29'::date + '10 years'::interval) = '1922-02-28'::date

returns true.

The inconsistency (or bug) resides in the non unique meaning of the interval handling with respect of summing and subtracting the same quantity. 

Again, the issue can be seen this way: adding the interval returned by 

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SELECT age('1922-02-28'::date, '1912-02-29'::date) to the original date. So:

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SELECT '1912-02-29'::date + '9 years 11 mons 28 days'::interval

returns:

(c) 1922-02-26 00:00:00

Here we loose 2 days even if we add the same interval used in (b)!

The bug basically consists of the vague meaning of “years” applied to leap years. It should be revised in order to be consistent and correct.