const before = { d: new Date().valueOf() }; console.log(before.d); // 1471490389847 (UNIXエポックからのミリ秒数) console.log(typeof before.d); // number const json = JSON.stringify(before); console.log(json) // {"d":1471490389847} const after = JSON.parse(json); console.log(after) // { d: 1471490389847 } console.log(typeof after.d) // number const d = new Date(after.d); console.log(d); // 2016-08-18T03:19:49.847Z