Calling a function inside of JSONB?
Sure, JSON doesn't support a special date type, (neither does JavaScript), JSONB only supports 


string
number
boolean
null


So just use string interpolation, it'll get stored the same way:

SELECT (
  '{"value": 5, "time": "' || now() || '" }'
)::JSONB;


If you're trying to simply get NOW() from the server in one call, you can certainly do that using jsonb_set, or jsonb concatenation (||). Here are the docs

SELECT '{"value": 5}'::JSONB || jsonb_build_object('time', now());


In a third party library this may look like..

INSERT INTO table (json)
VALUES ( ? || jsonb_build_object('time', now()) );


It's still ugly. I wouldn't store a datatime in a JSONB, unless I had to:


It will be slow.
It will take more space.


As an aside, one of the alternatives to JSONB that Pg could have chosen is BSON which doesn't store dates as strings..