How to convert timestamp with time zone to numeric (unix) in PostgreSQL?
To solve this, I looked here.

For example, I'm in a time zone which is at UTC + 1, 

so, 

SELECT CURRENT_TIMESTAMP AT TIME ZONE 'UTC';


gives:

2019-06-06 17:41:00.216647


which is 1 hour behind clock time because I'm on UTC + 1.

So, then from here, I got select extract(epoch from ts) from data

Combining the two gives:

SELECT EXTRACT(EPOCH FROM (CURRENT_TIMESTAMP AT TIME ZONE 'UTC'));


which for me gives:

date_part
1559843103.53928


YMMV - fiddle available here - put your timestampz into the formula for current_timestamp and you should be golden!

Interstingly, you can check your results here.

p.s. welcome to the forum! :-)