I need to convert timestamp to double precision without using EPOCH in postgresql

I need to convert timestamp to double precision without using EPOCH in
postgresql

I need to convert timestamp into string or anything at one place and
convert it back to timestamp at another place. So itried the following.
but i'm not getting a slight difference. When i convert it back to
timestamp last digit gets missing.
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2013-09-27
13:51:02.809501+05:30'); SELECT TIMESTAMP WITH TIME ZONE 'epoch' +
1380270062809.5 * INTERVAL '1 millisecond'; result is..."2013-09-27
13:51:02.8095+05:30"