how to add 'time with time zone' data types and 'interval' data types

hi all,

1st scenario: i have a table with two fields both of them ''time with time
zone'. How could I add them? i would like to have something like: SELECT
table.field1 + table.field2 as sum;
2nd scenario, similar but this time data types are 'interval': i have a
table with two fields both of them ''interval'. How could I add them? i
would like to have something like: SELECT table.field1 + table.field2 as
sum;

thanks for your hint,
John.