I presume it affects 5.3 too, but I didn't check.
The issue comes from the fact that date and datetime values like '2012-00-00' are vaguely supported (on one hand, MySQL docs say that ranges for month and day are 1..12 and 1..31, respectively; on the other hand, it only mentions that zero values for month and day are invalid for TIMESTAMP, but doesn't say anything for DATE/DATETIME).
So, this works, no errors or warnings:
Comparison works too:
Now, we add 1 day to the value. Again, no warnings or errors:
But the result is not what someone would expect:
In MySQL (5.6, did not try earlier) it is also not perfect: adddate turns the value to NULL, again without a warning. But it seems somewhat less strange than reducing it.