activerecord update_attributes have a strange action with date

I am using activerecord 2.1.0 and mysql,and with mysql 2.7 driver.
I have seen that when I use update_attributes,the date value have a
strange action.

Think we have a table Tests with
id int,
dt datetime

and when I use,
a[:dt] = "20081128"
test =

it ran ok,

but when I set the date value to "31000101"
the dt had been set to NULL.

anyone have same problem?