The way that I would do it, which is probably not the only way, would be to use the JavaScript tool box plug-in and use a workflow action. Using the results of the API returned data, I would use moment JS or Luckson JS to parse that time I am imagining you probably want that as a string and not a date object so it might be something like this in your JavaScript. Be sure to return a date.
var d = new Date(Results from Step 1’s api call * 1000)
d.setUTCSeconds(utcSeconds);
d;
I just realized that that’s not what you need. I was looking at that as if it was a epoch time. You need a text for matter. I believe I’ve seen a plug-in that would do this for you I’ll have to look around. In the meantime hopefully somebody else can be helpful!
A quick search revealed the following plug-in. It may be helpful but not exactly what you’re looking for.