const SiteClient = require('datocms-client').SiteClient;
const client = new SiteClient('YOUR-API-TOKEN');
const itemId = '34';
client.scheduledUnpublishing.create(itemId, {
  unpublishingScheduledAt: '2017-02-10T11:03:42.208Z'
})
.then((scheduledUnpublishing) => {
  console.log(scheduledUnpublishing);
})
.catch((error) => {
  console.error(error);
});

> node example.js
{
  "id": "34",
  "unpublishingScheduledAt": "2017-02-10T11:03:42.208Z",
  "item": "4235"
}