triggers - How to let MySQL NOW() function return the user time -

i ran tricky mysql issue.. our db using type datetime updated_at column, , late change timestamp. i'm trying add trigger update updated_at column on table changing. following did:

create trigger foo_schema.bar_datetime_trigger before update on foo_schema.bar each row set new.updated_at = now();

the now() function here returns db time; however, want set udpated_at user time (meaning time of user made changes table bar). possible handle in mysql?