Oracle 18c

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CREATE TABLE attr
    (attr_name varchar(30), attr_value varchar(30))
INSERT INTO attr
    (attr_name, attr_value)
VALUES
    ('createTimestamp', '1547250541000');
  
1 rows affected

INSERT INTO attr
    (attr_name, attr_value)
VALUES
    ('createTimestamp', '1547240431000');
  
1 rows affected

INSERT INTO attr
    (attr_name, attr_value)
VALUES
    ('createTimestamp', '1547240321000');
  
1 rows affected

 alter session set NLS_DATE_FORMAT = 'yyyy-mm-dd HH24:mi:ss'; 
SELECT trunc(estimatedenddate, 'hh') AS REG_DATE, count(*)
 FROM (select attr.attr_value,  TO_DATE((DATE '1970-01-01' + ( 1 / 24 / 60 / 60 / 1000) * attr.attr_value), 'yyyy-mm-dd HH24:mi:ss') AS estimatedenddate
 from  attr WHERE attr_name='createTimestamp')   GROUP BY  trunc(estimatedenddate, 'hh')
  ORDER BY REG_DATE DESC


REG_DATE
COUNT(*)


2019-01-11 23:00:00
1

2019-01-11 21:00:00
1

2019-01-11 20:00:00
1

REG_DATE	COUNT(*)
2019-01-11 23:00:00	1
2019-01-11 21:00:00	1
2019-01-11 20:00:00	1