@theshaden Welcome to the community 
Yes date format differs, and for your request do you see any format in your API reference/documentation?
Usually we can get the date using newDate() method or “moment”. And we need to format it before sending it to the headers.
const moment = require('moment');
pm.globals.set("timestamp", moment().format("MM/DD/YYYY hh:mm:ss"));
or
var hdate = new Date();
console.log(hdate);
Kindly provide more details/screenshots to get to understand the issue 