Closest datetime for PostgreSQL 9.5 - postgresql

Question

I'm using PostgreSQL 9.5
and I have a table like this:
CREATE TABLE tracks (
track bigserial NOT NULL,
time_track timestamp,
CONSTRAINT pk_aircraft_tracks PRIMARY KEY ( track )
);
I want to obtain track for the closest value of datetime by SELECT operator.
e.g, if I have:
track datatime
1 | 2016-12-01 21:02:47
2 | 2016-11-01 21:02:47
3 |2016-12-01 22:02:47
For input datatime 2016-12-01 21:00, the track is 2.
I foud out Is there a postgres CLOSEST operator? similar queston for integer.
But it is not working with datatime or PostgreSQL 9.5 :
SELECT * FROM
(
(SELECT time_track, track FROM tracks WHERE time_track >= now() ORDER BY time_track LIMIT 1) AS above
UNION ALL
(SELECT time_track, track FROM tracks WHERE time_track < now() ORDER BY time_track DESC LIMIT 1) AS below
)
ORDER BY abs(?-time_track) LIMIT 1;
The error:
ERROR: syntax error at or near "UNION"
LINE 4: UNION ALL

Answer 1

Track 1 is the closest to '2016-12-01 21:00':
with tracks(track, datatime) as (
values
(1, '2016-12-01 21:02:47'::timestamp),
(2, '2016-11-01 21:02:47'),
(3, '2016-12-01 22:02:47')
)
select *
from tracks
order by
case when datatime > '2016-12-01 21:00' then datatime - '2016-12-01 21:00'
else '2016-12-01 21:00' - datatime end
limit 1;
track | datatime
-------+---------------------
1 | 2016-12-01 21:02:47
(1 row)