The subtime function wasn't able to produce correct int representation of
its result. For constant expressions the Item_datetime_cache is used to
speedup evaluation and Item_datetime_cache expects underlying item to return
correct int representation of DATETIME value. These two factors combined led
to a wrong query result.

Now the Item_func_add_time has function val_datetime which performs the
calculation and saves result into given MYSQL_TIME struct, it also sets
null_value to appropriate value. val_int and val_str member functions
convert the result obtained from val_datetime to int or string respectively
and returns it.