https://twitter.com/SDRycroft/status/936537711279779841

gistfile1.txt

if((Date.now() / 1000 | 0) > pm.environment.get("oauthTokenExpires")) {
    pm.sendRequest({
        url: "{{baseUrl}}/token",
        method: 'POST',
        header: 'Content-Type: application/json',
        body: {
            mode: 'raw',
            raw: JSON.stringify({
                grant_type: "client_credentials",
                client_id:"{{clientIdentifier}}",
                client_secret:"{{clientSecret}}"
            })
        }
    }, function (err, response) {
        pm.environment.set("oauthTokenExpires", (Date.now() / 1000 | 0) + response.json().expires_in)
        pm.environment.set("oauthToken", response.json().access_token);
    });
}