specifying DEFAULT

This was not specific to datetime.  When there is no default value 
for a column, and the user inserted DEFAULT, we would write 
uninitialized memory to the table.  

Now, insist on writing a default value, a zero-ish value, the same 
one that comes from inserting NULL into a not-NULL field.

(This is, at best, really strange behavior that comes from allowing 
sloppy usage, and serves as a good reason always to run one's server 
in a strict SQL mode.)