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?