[SQL] Substracting Date field from integer value in another field

Getting below error.

ERROR:  operator does not exist: timestamp with time zone - bigint

LINE 65: ACS_TIMESTAMP-acct_session_time/(24*60*60) ELSE ACS_TIMESTAM..

I have a table with one column as  timestamp (Logon Time), another column as integer(seconds).

I would like to deduct the logon time(timestamp) – integer field(seconds) to get the final value in TIMESTAMP format.

SELECT ACS_TIMESTAMP - acct_session_time   FROM RADIUS_ACC_48

ERROR:  operator does not exist: timestamp with time zone - bigint

LINE 65: ACS_TIMESTAMP-acct_session_time/(24*60*60) ELSE ACS_TIMESTAM..

Please let me know the syntax for that.