Updating tuples based on max time

I keep crashing my backend doing the following
UPDATE logins SET
    logoff_time='now'
WHERE
    login_time=max(login_time)
AND
    account_name='tempuser'
;

login_time is type 'datetime'

As a side note, I used this method because the following didn't work
UPDATE logins SET
    logoff_time='now'
WHERE
    id=(SELECT DISTINCT id FROM logins
            WHERE
                login_time IS NOT NULL
            AND
                account_name='tempuser'
        )
;

Point is, what is the best method to update a record based on it's
datetime being the most recent?