MySQL 8.0

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CREATE TABLE Chron (id INT, `timestamp` TIMESTAMP);
INSERT INTO Chron (id, `timestamp`)
VALUES
    (11, '2018-01-01'),
    (12, '2018-01-02'),
    (13, '2000-01-01'),
    (14, '2000-01-01'),
    (15, '2018-01-05'),
    (17, '2018-01-07'),
    (18, '2000-01-01'),
    (19, '2018-01-09')
;
SELECT * FROM Chron;


id
timestamp


11
2018-01-01 00:00:00

12
2018-01-02 00:00:00

13
2000-01-01 00:00:00

14
2000-01-01 00:00:00

15
2018-01-05 00:00:00

17
2018-01-07 00:00:00

18
2000-01-01 00:00:00

19
2018-01-09 00:00:00


SET @D = '2000-01-01';
SELECT l.id, l.`timestamp`, /* l.l, h.id, h.`timestamp`, h.h, */ CASE WHEN l.id+l.id > l.l+h.h THEN h.h ELSE l.l END closest
FROM
(SELECT @tmp := 0) dummy,
(
SELECT Chron.id, Chron.`timestamp`, @tmp := CASE WHEN Chron.`timestamp` = @D THEN @tmp ELSE Chron.id END l
FROM Chron
ORDER BY Chron.id ASC
) l,
(
SELECT Chron.id, Chron.`timestamp`, @tmp := CASE WHEN Chron.`timestamp` = @D THEN @tmp ELSE Chron.id END h
FROM Chron
ORDER BY Chron.id DESC
) h
WHERE l.id = h.id


id
timestamp
closest


11
2018-01-01 00:00:00
11

12
2018-01-02 00:00:00
12

13
2000-01-01 00:00:00
12

14
2000-01-01 00:00:00
15

15
2018-01-05 00:00:00
15

17
2018-01-07 00:00:00
17

18
2000-01-01 00:00:00
17

19
2018-01-09 00:00:00
19

id	timestamp
11	2018-01-01 00:00:00
12	2018-01-02 00:00:00
13	2000-01-01 00:00:00
14	2000-01-01 00:00:00
15	2018-01-05 00:00:00
17	2018-01-07 00:00:00
18	2000-01-01 00:00:00
19	2018-01-09 00:00:00