JSFiddle - React, Tailwind, and code Playground

JavaScript

var data = [{
    "Active":true,
    "EndDate":"\/Date(-62135578800000-0500)\/",
    "StartDate":"\/Date(1280635200000-0400)\/"
}];

var dateParts = data[0].StartDate.match(/\((.*)([+-])(..)(..)\)/);
var dateObj = new Date(
    /* timestamp in milliseconds */ Number(dateParts[1]) +
    /* sign of timezone offset */ Number(dateParts[2] + '1') *
    /* hours and minutes offset */ (36e5 * dateParts[3] + 6e4 * dateParts[4])
);

var dateMMDDYYYY = [dateObj.getUTCMonth() + 1,
                    dateObj.getUTCDate(),
                    dateObj.getUTCFullYear()].join('/');

alert(dateObj);
alert(dateMMDDYYYY);