How to match different formats in teradata?

Question

SQL

Hello everyone, in order to parse, I need to find the completion dates of the code in the database.
There is a start date of the action in the date format, and how many days it worked in the integer format (if I'm not confused, there is no access now).
I need to determine the end date of a job by bringing together the start and end of the job.
Tried the following solutions:

startdate + codeday*(INTERVAL '1' DAY)
startdate+codeday
to_char(startdate+codeday)

.
They do not help, give errors: startdate + codeday*(INTERVAL '1' DAY)- "interval filed overflow".
to_char(startdate+codeday)- execute function access to syslib