postman_pre_request_script.js

var date = new Date();
var dateString = [date.getFullYear(), 
                  ('0' + (date.getMonth() + 1)).slice(-2),
                  ('0' + date.getDate()).slice(-2)].join('/');
dateString += ' ';
dateString += [('0' + date.getUTCHours()).slice(-2),
               ('0' + date.getUTCMinutes()).slice(-2),
               ('0' + date.getUTCSeconds()).slice(-2)].join(':');
dateString += ' +' + ((date.getTimezoneOffset() / 60) * -100);

postman.setGlobalVariable("yourVariable", dateString);