Not able to create OAuth Token using Private Key for Salesforce Einstein

I've been following the documentation on how to create an OAuth token for Salesforce Einstein. I have my private key and able to create the JWT token (code on bottom) yet when I try the CURL command as outlined in the documentation (step 4) with the token I've retrieved I get INVALID JWT Assertion error.

Documentation : https://metamind.readme.io/docs/generate-an-oauth-token-using-your-key

This is how I'm creating the signed JWT

require('dotenv').config();
const jwtToken = require('jsonwebtoken');

const payload = 
{
 "sub": "<EINSTEIN_PLATFORM_SERVICES_USERNAME>",
 "aud": "https://api.einstein.ai/v2/oauth2/token",
 "exp": <EXPIRATION_SECONDS_IN_UNIX_TIME>
}

const token = jwtToken.sign(payload, process.env.EINSTEIN_KEY, {algorithm : 'RS256'});

Any idea what I'm doing wrong?

It took me a while to figure this out...the problem was the UNIX time I gave was something of the year 1970 -_- so of course, since the JWT token expired I was not able to retrieve an access token from Salesforce Einstein.

I suggest using the following resource to get the correct expiration time in UNIX seconds : https://www.epochconverter.com/.

Here's my final code for generating a JWT Assertion String :

require('dotenv').config()
const fs = require('fs);
const jwt = require('jsonwebtoken');

let privateKey = fs.readFileSync(process.env.PRIVATE_KEY);

const header = {
  algorithm: "RS256"
}

const payload = {
 "sub": "<EINSTEIN_PLATFORM_SERVICES_USERNAME>",
 "aud": "https://api.einstein.ai/v2/oauth2/token",
 "exp": 1907891585 //Epoch timestamp of 6/17/2030
}

let token = jwt.sign(payload, privateKey, header);

With this code, I was able to retrieve an access token!