(based on a find by Ruud van Tol, and several Twitter contributions)

Ruud commented on our DST discussion with

mysql> SELECT 
'2019-02-28 12:34:56'+ INTERVAL 1 YEAR + INTERVAL 1 DAY as a, 
'2019-02-28 12:34:56'+ INTERVAL 1 DAY + INTERVAL 1 YEAR  as b\G
a: 2020-02-29 12:34:56
b: 2020-03-01 12:34:56

2019 is a year before a leap year. Adding (left to right) a year brings us to 2020-02-28, and then adding a day makes this 2020-02-29, because it’s a leap year.

On the other hand, adding a day first makes it 2019-03-01, and then adding a year makes it 2020-03-01, a different result.

Clearly, addition is not commutative on dates, and having a two step interval addition is breaking expectations here.

Compound Interval Notation

MySQL is offering a bit of syntax for compound intervals. You can look it up in the manual.

To write up compound intervals, there is a select number of unit names that are actually allowed, and a special expression syntax for each one.

For example, you can + interval 12:23:56.789 hour_microsecond, or + interval 01-01 year_month. You can’t jump a year and a day, because there is no unit for that. Instead you have to write this down in MySQL in a two step interval addition and suddenly order matters.

If you think that this is a cumbersome solution and a cumbersome syntax, you will see me nodding in agreement.

Other databases

SQLite

Mohammed S. Al Sahaf contributes this syntax for SQLite:

sqlite> select
  date('2019-02-28', '+1 year', '+1 day') as a,
  date('2019-02-28', '+1 day', '+1 year') as b;
a           b
----------  ----------
2020-02-29  2020-03-01

confirming that this is not a problem specific to MySQL.

Postgres

Andreas Scherbaum demonstrates the more generic Postgres Syntax:

pgsql> SELECT 
'2019-02-28 12:34:56'::TIMESTAMP + INTERVAL '1 year 1 day' as a,
'2019-02-28 12:34:56'::TIMESTAMP + INTERVAL '1 day 1 year' as b;

a: 2020-02-29T12:34:56Z	
b: 2020-02-29T12:34:56Z	

This works, because Postgres offers a syntax for a single interval that can combine arbitrary units. So internally both times it’s the same order of operations (a span of a year and a day) in a single interval.

The two-step operation gives the same result as MYSQL.

pgsql> SELECT 
'2019-02-28 12:34:56'::TIMESTAMP + INTERVAL '1 year' + INTERVAL '1 day' as a,
'2019-02-28 12:34:56'::TIMESTAMP + INTERVAL '1 day' + INTERVAL '1 year' as b;

a: 2020-02-29T12:34:56Z	
b: 2020-03-01T12:34:56Z

Cockroach

Mohammed also demonstrates that Cockroach has the same syntax that allows Postgres to make the calculation in a single step:

Cockroach> select
'2019-02-28 12:34:56'::TIMESTAMP + INTERVAL '1 year 1 day' as a,
'2019-02-28 12:34:56'::TIMESTAMP + INTERVAL '1 day 1 year' as b;
              a             |             b
----------------------------+----------------------------
  2020-02-29 12:34:56+00.00 | 2020-02-29 12:34:56+00.00

What to do about it?

I do not think that the different results for “a day and a year” and “a year and a day” for leap years in current MySQL are a bug, but they are certainly unexpected.

I do think the current syntax for compound intervals that MySQL has is cumbersome and limited. It is also different from Postgres and Cockroach.

Picking up Postgres/Cockroach interval notation would allow MySQL to get rid of “day_microsecond” and all the special expression syntaxes. It would also allow it to write arbitrary intervals with a much simpler notation.

So, MySQL, please consider

mysql> select '2019-02-29 12:34:56' + interval '1 day 1 year' as a;

a: 2020-02-29 12:34:56

sort it internally into a canonical expression and make it possible to jump to ‘2020-02-29 12:34:56’ in a single interval jump.

Until then, make sure that you manually order your chained intervals into the proper sequence of steps. And if it is not from large to small, prepare to be surprised.