Difference between TIMESTAMPS in Teradata

The difference between two TIMESTAMP types is an Interval type.

Consider a table like this:
(phone_no CHARACTER(10)
,start_time TIMESTAMP(0)
,end_time TIMESTAMP(0));

The difference can be calculated with Interval unit DAY TO SECOND like below.
SELECT (end_time - start_time) DAY(4) TO SECOND
FROM customer;

The DAY(4) specifies four digits of precision, and allows for a maximum of 9999 days, or approximately 27 years.

A typical result looks like below,
4 15:30:28.34000