length.

When temporary field created for DATE(LEFT(column,8)) expression, max_length
value is taken from Item_date_typecast, and it is getting it from underlaid
Item_func_left and it's max_length is 8 in given expression. And all this
results in stripping last 2 digits.

To Item_date_typecast class added its own fix_length_and_dec() function 
that sets max_length value to 10, which is proper for DATE field.