HSQLDB (HyperSQL) - how to get UNIX timestamp as a number with ms precision

Question

I'm trying to get a Java-style long timestamp, that is, UNIX timestamp with millisecond precision * 1000, which fits into a non-floating-point type (BIGINT).

I didn't find a way to get it straight from some function, like CURRENT_TIMESTAMP, unless I was okay with formatting like 20181010123059123.

So I found that this would give me something that looks like a number:

(CURRENT_TIMESTAMP - TIMESTAMP '2018-01-01 00:00:00') SECOND TO SECOND
-- Gives: 23649115.452000

Note that I am substracting 2018-... since I only care about the delta, not the absolute date.

I am not sure if this is the simples way.
Turns out the type of this is INTERVAL, so I need to convert:

CAST(
    (CURRENT_TIMESTAMP - TIMESTAMP '2018-01-01 00:00:00') SECOND TO SECOND
    AS DECIMAL(15,3)
)
-- Gives 23649115.000

Now the issue is, the precision is lost.
So I wonder: Where is the .452 lost and how can I keep it? This is what the manual says:

An interval value can be cast to a numeric type. In this case the interval value is first converted to a single-field INTERVAL type with the same field as the least significant filed of the interval value. The value is then converted to the target type. For example CAST (INTERVAL '1-11' YEAR TO MONTH AS INT) evaluates to INTERVAL '23' MONTH, and then 23.

And the ultimate question is:
How can I get UNIX timestamp-like number of milliseconds since some moment, e.g. UNIX epoch start?

My current whole SQL:

SELECT  
(CURRENT_TIMESTAMP - TIMESTAMP '2018-01-01 00:00:00') SECOND TO SECOND,
FLOOR(
     CAST(
      (CURRENT_TIMESTAMP - TIMESTAMP '2018-01-01 00:00:00') SECOND TO SECOND
       AS DECIMAL(15,3)
     ) * 1000
) 
FROM (VALUES(0));
-- Gives: 23649115.452000 | 23649115000

Answer 1

Turns out there's UNIX_MILLIS which I overlooked (because the broken PDF format manual prevents proper searching).

SELECT  UNIX_MILLIS() FROM (VALUES(0)) 

Which renders my attempts above a nice excercise in intervals.

I still wonder, how should I CAST an interval to retain the milliseconds part.