Can you confirm the URL you’re POST’ing to, as to my knowledge there isn’t a /helix/token endpoint, you should be using https://id.twitch.tv/oauth2/token for token requests, and from my tests I haven’t been able to reproduce an expiration that’s a UNIX timestamp, for me it’s in seconds (from the tries in the last few minutes the expiration has varied between 54 and 63 days).