What is the behavior for the minus operator between two datetimes in MySQL?

Question

The difference between to datetimes is the number of seconds between them. This seems to work only if the datetimes occur in the same hour.

Why is this?

mysql> update events set created_at = "2011-04-13 15:59:59", fulfilled_at ="2011-04-13 16:00:00" where id = 1;
mysql> select fulfilled_at - created_at, timediff(fulfilled_at, created_at) from events where id = 1;
+---------------------------+------------------------------------+
| fulfilled_at - created_at | timediff(fulfilled_at, created_at) |
+---------------------------+------------------------------------+
|               4041.000000 | 00:00:01                           |
+---------------------------+------------------------------------+

I know I should be using timediff, but I'm just curious why I'm seeing this or if it's documented somewhere.

Answer 1

MySQL is just converting strings into numbers as best it can, so that it can do the mathematical operation on them. In this case, its just stripping out all of the non numerical colons, dashes and spaces.

Try this:

SELECT (20110413155959 - 20110413160000) AS dates;

Your dates, without all the stuff that stops them being numbers - the result is -4041