SELECT sysdate - 1 FROM dual;                 -- Yesterday    12/01/2021 14:27:27
SELECT trunc(sysdate) - 1 FROM dual;         -- Yesterday    12/01/2021 00:00:00
SELECT sysdate + INTERVAL '20' DAY FROM DUAL;        -- 20 days ahead (other way)
SELECT sysdate - 1 / 24 FROM dual;                    -- 1 hour ago
SELECT sysdate - 1 / 24 / 60 FROM dual;                -- 1 minute ago
SELECT add_months(trunc(sysdate), -1) FROM dual;       -- 1 month ago (no time)
SELECT trunc(sysdate) - 30 FROM dual;                -- 30 days ago (no time)
SELECT add_months(trunc(sysdate), -12*2) FROM dual;    -- 2 years ago (no time)

-- ⇓ Test it ⇓ (Fiddle source link)