Postgres 12

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CREATE TABLE test (data JSONB);
INSERT INTO test VALUES
('{"started_at":"2020-01-01T01:02:03.456","finished_at":"2020-02-03T12:23:34.987"}');
SELECT * FROM test;
1 rows affected

data

{"started_at": "2020-01-01T01:02:03.456", "finished_at": "2020-02-03T12:23:34.987"}

UPDATE test
SET data = REGEXP_REPLACE(data::TEXT, '(\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d\.\d{3})', '\1+03', 'g')::JSONB
WHERE data::TEXT !~ '\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d\.\d{3}\+\d\d';
SELECT * FROM test;
1 rows affected

data

{"started_at": "2020-01-01T01:02:03.456+03", "finished_at": "2020-02-03T12:23:34.987+03"}

INSERT INTO test VALUES
('{"started_at":"2021-01-01T01:02:03.456","finished_at":"2021-02-03T12:23:34.987"}');
SELECT * FROM test;
UPDATE test
SET data = REGEXP_REPLACE(data::TEXT, '(\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d\.\d{3})', '\1+03', 'g')::JSONB
WHERE data::TEXT !~ '\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d\.\d{3}\+\d\d';
SELECT * FROM test;
1 rows affected

data

{"started_at": "2020-01-01T01:02:03.456+03", "finished_at": "2020-02-03T12:23:34.987+03"}

{"started_at": "2021-01-01T01:02:03.456", "finished_at": "2021-02-03T12:23:34.987"}

1 rows affected

data

{"started_at": "2020-01-01T01:02:03.456+03", "finished_at": "2020-02-03T12:23:34.987+03"}

{"started_at": "2021-01-01T01:02:03.456+03", "finished_at": "2021-02-03T12:23:34.987+03"}