convert time format retuned by a select query

I would like to subtract two columns - create_time and update_time of
'timestamp without time zone' type and then order results based on
subtraction-result (time interval). I checked
http://www.postgresql.org/docs/current/static/functions-datetime.html ,
however these examples didn't work with actual column values. e.g.something
like:
'select epoch (update_time) - epoch (create_time) AS Column1 from users
order by Column1; '

Any hints or documentation liks will be really nice. Thanks.

jM