How to format timestamp into string

Greeting Gurus,
I have studied and search through the mighty web but no luck. My intention is to allow format a proper timestamp string that supported by the API and put under request URL, also perform basic time caculation (e.g. timezone conversion, +3 hrs to system time) But as FQL quite limited and doesn’t allow me to type JS script, i find it hard to achieve my goal in postman flow. Thanks for the pointers