Postgres 11

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create table t1(id int, client_id int, dateval date)

CREATE TABLE

insert into t1 values
(1,1001,'2008-11-26 00:00:00'),
(2,2001,'2008-09-01 00:00:00'),
(3,2001,'2008-08-31 00:00:00'),
(4,1001,'2007-08-26 00:00:00'),
(5,3001,'2003-11-26 00:00:00'),
(4,1001,'2003-05-05 00:00:00')

INSERT 0 6

select id, client_id,date_part('day',age(dateval, prev_Date)) as datediff
from
(
select id, client_id, dateval, lag(dateval) over(partition by client_id order by id) as prev_Date
from t1
)A 
where date_part('day',age(dateval, prev_Date))>=30

id
client_id
datediff

SELECT 0

select to_timestamp('2019-09-20 13:59', 'YYYY-MM-DD HH24:MI:SS')

to_timestamp

2019-09-20 13:59:00+01

SELECT 1