Is there a script to convert Timestamp (UNIX) time to ISO 8601?

Question

JavaScript

Hello!
I was puzzled by the micro-markup and ran into trouble - I can only receive the date and time of the news in the Timestamp format - 1503300202 or in the usual one - 08/21/2017, but neither one nor the other is suitable. I would like to give the date of the material for search engines in the format ISO 8601 - 2017-08-21T11:30:30+03:00

<meta itemprop="datePublished" content="$TIMESTAMP$">

To save myself from unnecessary manual work, I would like to fasten some light script that would translate $TIMESTAMP$ into ISO 8601 and substitute it into content="...". Is there something similar? If so, I'd be grateful for an example.
Thank you!