我认为curtime（），now（）和current_timestamp是MySql中有效的默认日期时间值？

I'm using the latest version of MySql and I tried altering a table to use curtime() or now() or current_timestamp as the default value for a datetime column (created_at, updated_at). For all three, the alter failed, saying these are invalid default values. Any ideas what's an appropriate default value to generate the current time in a datetime field?I'm using the latest version of MySql and I tri