Conversion of a column from Integer format type to 'timestamp with time zone'

Hi, there is a column 'created' in the 'customers' table which is actually
supposed to be a 'timestamp with time zone' column but stored in this
Integer format. How do I typecast this to 'timestamp with time zone' format?

select created from customers limit 3;
created
---------------
1521521848681
1508995056368
1521559994299
(3 rows)

Thanks,
Raj Kumar Narendiran.