Just pass the time as is from the API to the discord Webhook/Embed, then Discord will display the embed with the discord client users timezone anyway.
No real need for you to convert it before passing it to Discord, as you can just pass UTC to discord and discord client will convert.
nodeJS I used to recommend moment.js (https://momentjs.com/) but they are closing in favour of Luxon https://moment.github.io/luxon/