PostgreSQL: change date by the random number of days

select date(now() + trunc(random()  * 20) * '1 day'::interval);

Any existing date column can also be used in place of now()

See: https://www.postgresql.org/docs/current/functions-datetime.html

How random? For example - if you want random data from last year:

select cast( now() - '1 year'::interval * random()  as date );